Index: Python/ceval.c
===================================================================
--- Python/ceval.c	(revision 74298)
+++ Python/ceval.c	(working copy)
@@ -2164,6 +2164,25 @@
 			}
 			break;
 
+		case BUILD_SET:
+			x = PySet_New(NULL);
+			if (x != NULL) {
+				for (; --oparg >= 0;) {
+					w = POP();
+					if (err == 0)
+						err = PySet_Add(x, w);
+					Py_DECREF(w);
+				}
+				if (err != 0) {
+					Py_DECREF(x);
+					break;
+				}
+				PUSH(x);
+				continue;
+			}
+			break;
+
+
 		case BUILD_MAP:
 			x = _PyDict_NewPresized((Py_ssize_t)oparg);
 			PUSH(x);
Index: Python/graminit.c
===================================================================
--- Python/graminit.c	(revision 74298)
+++ Python/graminit.c	(working copy)
@@ -1548,258 +1548,298 @@
 	{2, arcs_72_4},
 };
 static arc arcs_73_0[1] = {
-	{161, 1},
+	{28, 1},
 };
-static arc arcs_73_1[1] = {
+static arc arcs_73_1[3] = {
+	{23, 2},
+	{29, 3},
+	{0, 1},
+};
+static arc arcs_73_2[1] = {
+	{28, 4},
+};
+static arc arcs_73_3[2] = {
+	{28, 5},
+	{0, 3},
+};
+static arc arcs_73_4[2] = {
+	{29, 6},
+	{0, 4},
+};
+static arc arcs_73_5[2] = {
+	{29, 3},
+	{0, 5},
+};
+static arc arcs_73_6[2] = {
+	{28, 7},
+	{0, 6},
+};
+static arc arcs_73_7[1] = {
+	{23, 2},
+};
+static state states_73[8] = {
+	{1, arcs_73_0},
+	{3, arcs_73_1},
+	{1, arcs_73_2},
+	{2, arcs_73_3},
+	{2, arcs_73_4},
+	{2, arcs_73_5},
+	{2, arcs_73_6},
+	{1, arcs_73_7},
+};
+static arc arcs_74_0[1] = {
+	{162, 1},
+};
+static arc arcs_74_1[1] = {
 	{21, 2},
 };
-static arc arcs_73_2[2] = {
+static arc arcs_74_2[2] = {
 	{13, 3},
 	{23, 4},
 };
-static arc arcs_73_3[2] = {
+static arc arcs_74_3[2] = {
 	{9, 5},
 	{15, 6},
 };
-static arc arcs_73_4[1] = {
+static arc arcs_74_4[1] = {
 	{24, 7},
 };
-static arc arcs_73_5[1] = {
+static arc arcs_74_5[1] = {
 	{15, 6},
 };
-static arc arcs_73_6[1] = {
+static arc arcs_74_6[1] = {
 	{23, 4},
 };
-static arc arcs_73_7[1] = {
+static arc arcs_74_7[1] = {
 	{0, 7},
 };
-static state states_73[8] = {
-	{1, arcs_73_0},
-	{1, arcs_73_1},
-	{2, arcs_73_2},
-	{2, arcs_73_3},
-	{1, arcs_73_4},
-	{1, arcs_73_5},
-	{1, arcs_73_6},
-	{1, arcs_73_7},
+static state states_74[8] = {
+	{1, arcs_74_0},
+	{1, arcs_74_1},
+	{2, arcs_74_2},
+	{2, arcs_74_3},
+	{1, arcs_74_4},
+	{1, arcs_74_5},
+	{1, arcs_74_6},
+	{1, arcs_74_7},
 };
-static arc arcs_74_0[3] = {
-	{162, 1},
+static arc arcs_75_0[3] = {
+	{163, 1},
 	{30, 2},
 	{31, 3},
 };
-static arc arcs_74_1[2] = {
+static arc arcs_75_1[2] = {
 	{29, 4},
 	{0, 1},
 };
-static arc arcs_74_2[1] = {
+static arc arcs_75_2[1] = {
 	{28, 5},
 };
-static arc arcs_74_3[1] = {
+static arc arcs_75_3[1] = {
 	{28, 6},
 };
-static arc arcs_74_4[4] = {
-	{162, 1},
+static arc arcs_75_4[4] = {
+	{163, 1},
 	{30, 2},
 	{31, 3},
 	{0, 4},
 };
-static arc arcs_74_5[2] = {
+static arc arcs_75_5[2] = {
 	{29, 7},
 	{0, 5},
 };
-static arc arcs_74_6[1] = {
+static arc arcs_75_6[1] = {
 	{0, 6},
 };
-static arc arcs_74_7[2] = {
-	{162, 5},
+static arc arcs_75_7[2] = {
+	{163, 5},
 	{31, 3},
 };
-static state states_74[8] = {
-	{3, arcs_74_0},
-	{2, arcs_74_1},
-	{1, arcs_74_2},
-	{1, arcs_74_3},
-	{4, arcs_74_4},
-	{2, arcs_74_5},
-	{1, arcs_74_6},
-	{2, arcs_74_7},
+static state states_75[8] = {
+	{3, arcs_75_0},
+	{2, arcs_75_1},
+	{1, arcs_75_2},
+	{1, arcs_75_3},
+	{4, arcs_75_4},
+	{2, arcs_75_5},
+	{1, arcs_75_6},
+	{2, arcs_75_7},
 };
-static arc arcs_75_0[1] = {
+static arc arcs_76_0[1] = {
 	{28, 1},
 };
-static arc arcs_75_1[3] = {
+static arc arcs_76_1[3] = {
 	{157, 2},
 	{27, 3},
 	{0, 1},
 };
-static arc arcs_75_2[1] = {
+static arc arcs_76_2[1] = {
 	{0, 2},
 };
-static arc arcs_75_3[1] = {
+static arc arcs_76_3[1] = {
 	{28, 2},
 };
-static state states_75[4] = {
-	{1, arcs_75_0},
-	{3, arcs_75_1},
-	{1, arcs_75_2},
-	{1, arcs_75_3},
+static state states_76[4] = {
+	{1, arcs_76_0},
+	{3, arcs_76_1},
+	{1, arcs_76_2},
+	{1, arcs_76_3},
 };
-static arc arcs_76_0[2] = {
+static arc arcs_77_0[2] = {
 	{156, 1},
-	{164, 1},
+	{165, 1},
 };
-static arc arcs_76_1[1] = {
+static arc arcs_77_1[1] = {
 	{0, 1},
 };
-static state states_76[2] = {
-	{2, arcs_76_0},
-	{1, arcs_76_1},
+static state states_77[2] = {
+	{2, arcs_77_0},
+	{1, arcs_77_1},
 };
-static arc arcs_77_0[1] = {
+static arc arcs_78_0[1] = {
 	{96, 1},
 };
-static arc arcs_77_1[1] = {
+static arc arcs_78_1[1] = {
 	{61, 2},
 };
-static arc arcs_77_2[1] = {
+static arc arcs_78_2[1] = {
 	{85, 3},
 };
-static arc arcs_77_3[1] = {
+static arc arcs_78_3[1] = {
 	{105, 4},
 };
-static arc arcs_77_4[2] = {
-	{163, 5},
+static arc arcs_78_4[2] = {
+	{164, 5},
 	{0, 4},
 };
-static arc arcs_77_5[1] = {
+static arc arcs_78_5[1] = {
 	{0, 5},
 };
-static state states_77[6] = {
-	{1, arcs_77_0},
-	{1, arcs_77_1},
-	{1, arcs_77_2},
-	{1, arcs_77_3},
-	{2, arcs_77_4},
-	{1, arcs_77_5},
+static state states_78[6] = {
+	{1, arcs_78_0},
+	{1, arcs_78_1},
+	{1, arcs_78_2},
+	{1, arcs_78_3},
+	{2, arcs_78_4},
+	{1, arcs_78_5},
 };
-static arc arcs_78_0[1] = {
+static arc arcs_79_0[1] = {
 	{92, 1},
 };
-static arc arcs_78_1[1] = {
+static arc arcs_79_1[1] = {
 	{106, 2},
 };
-static arc arcs_78_2[2] = {
-	{163, 3},
+static arc arcs_79_2[2] = {
+	{164, 3},
 	{0, 2},
 };
-static arc arcs_78_3[1] = {
+static arc arcs_79_3[1] = {
 	{0, 3},
 };
-static state states_78[4] = {
-	{1, arcs_78_0},
-	{1, arcs_78_1},
-	{2, arcs_78_2},
-	{1, arcs_78_3},
+static state states_79[4] = {
+	{1, arcs_79_0},
+	{1, arcs_79_1},
+	{2, arcs_79_2},
+	{1, arcs_79_3},
 };
-static arc arcs_79_0[2] = {
+static arc arcs_80_0[2] = {
 	{157, 1},
-	{166, 1},
+	{167, 1},
 };
-static arc arcs_79_1[1] = {
+static arc arcs_80_1[1] = {
 	{0, 1},
 };
-static state states_79[2] = {
-	{2, arcs_79_0},
-	{1, arcs_79_1},
+static state states_80[2] = {
+	{2, arcs_80_0},
+	{1, arcs_80_1},
 };
-static arc arcs_80_0[1] = {
+static arc arcs_81_0[1] = {
 	{96, 1},
 };
-static arc arcs_80_1[1] = {
+static arc arcs_81_1[1] = {
 	{61, 2},
 };
-static arc arcs_80_2[1] = {
+static arc arcs_81_2[1] = {
 	{85, 3},
 };
-static arc arcs_80_3[1] = {
+static arc arcs_81_3[1] = {
 	{107, 4},
 };
-static arc arcs_80_4[2] = {
-	{165, 5},
+static arc arcs_81_4[2] = {
+	{166, 5},
 	{0, 4},
 };
-static arc arcs_80_5[1] = {
+static arc arcs_81_5[1] = {
 	{0, 5},
 };
-static state states_80[6] = {
-	{1, arcs_80_0},
-	{1, arcs_80_1},
-	{1, arcs_80_2},
-	{1, arcs_80_3},
-	{2, arcs_80_4},
-	{1, arcs_80_5},
+static state states_81[6] = {
+	{1, arcs_81_0},
+	{1, arcs_81_1},
+	{1, arcs_81_2},
+	{1, arcs_81_3},
+	{2, arcs_81_4},
+	{1, arcs_81_5},
 };
-static arc arcs_81_0[1] = {
+static arc arcs_82_0[1] = {
 	{92, 1},
 };
-static arc arcs_81_1[1] = {
+static arc arcs_82_1[1] = {
 	{106, 2},
 };
-static arc arcs_81_2[2] = {
-	{165, 3},
+static arc arcs_82_2[2] = {
+	{166, 3},
 	{0, 2},
 };
-static arc arcs_81_3[1] = {
+static arc arcs_82_3[1] = {
 	{0, 3},
 };
-static state states_81[4] = {
-	{1, arcs_81_0},
-	{1, arcs_81_1},
-	{2, arcs_81_2},
-	{1, arcs_81_3},
+static state states_82[4] = {
+	{1, arcs_82_0},
+	{1, arcs_82_1},
+	{2, arcs_82_2},
+	{1, arcs_82_3},
 };
-static arc arcs_82_0[1] = {
+static arc arcs_83_0[1] = {
 	{28, 1},
 };
-static arc arcs_82_1[2] = {
+static arc arcs_83_1[2] = {
 	{29, 0},
 	{0, 1},
 };
-static state states_82[2] = {
-	{1, arcs_82_0},
-	{2, arcs_82_1},
+static state states_83[2] = {
+	{1, arcs_83_0},
+	{2, arcs_83_1},
 };
-static arc arcs_83_0[1] = {
+static arc arcs_84_0[1] = {
 	{21, 1},
 };
-static arc arcs_83_1[1] = {
+static arc arcs_84_1[1] = {
 	{0, 1},
 };
-static state states_83[2] = {
-	{1, arcs_83_0},
-	{1, arcs_83_1},
+static state states_84[2] = {
+	{1, arcs_84_0},
+	{1, arcs_84_1},
 };
-static arc arcs_84_0[1] = {
-	{168, 1},
+static arc arcs_85_0[1] = {
+	{169, 1},
 };
-static arc arcs_84_1[2] = {
+static arc arcs_85_1[2] = {
 	{9, 2},
 	{0, 1},
 };
-static arc arcs_84_2[1] = {
+static arc arcs_85_2[1] = {
 	{0, 2},
 };
-static state states_84[3] = {
-	{1, arcs_84_0},
-	{2, arcs_84_1},
-	{1, arcs_84_2},
+static state states_85[3] = {
+	{1, arcs_85_0},
+	{2, arcs_85_1},
+	{1, arcs_85_2},
 };
-static dfa dfas[85] = {
+static dfa dfas[86] = {
 	{256, "single_input", 0, 3, states_0,
-	 "\004\050\060\000\000\000\000\124\360\024\114\220\023\040\010\000\200\041\044\015\002\001"},
+	 "\004\050\060\000\000\000\000\124\360\024\114\220\023\040\010\000\200\041\044\015\004\002"},
 	{257, "file_input", 0, 2, states_1,
-	 "\204\050\060\000\000\000\000\124\360\024\114\220\023\040\010\000\200\041\044\015\002\001"},
+	 "\204\050\060\000\000\000\000\124\360\024\114\220\023\040\010\000\200\041\044\015\004\002"},
 	{258, "eval_input", 0, 3, states_2,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
 	{259, "decorator", 0, 7, states_3,
@@ -1819,11 +1859,11 @@
 	{266, "fplist", 0, 3, states_10,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"},
 	{267, "stmt", 0, 2, states_11,
-	 "\000\050\060\000\000\000\000\124\360\024\114\220\023\040\010\000\200\041\044\015\002\001"},
+	 "\000\050\060\000\000\000\000\124\360\024\114\220\023\040\010\000\200\041\044\015\004\002"},
 	{268, "simple_stmt", 0, 4, states_12,
-	 "\000\040\040\000\000\000\000\124\360\024\114\000\000\040\010\000\200\041\044\015\000\001"},
+	 "\000\040\040\000\000\000\000\124\360\024\114\000\000\040\010\000\200\041\044\015\000\002"},
 	{269, "small_stmt", 0, 2, states_13,
-	 "\000\040\040\000\000\000\000\124\360\024\114\000\000\040\010\000\200\041\044\015\000\001"},
+	 "\000\040\040\000\000\000\000\124\360\024\114\000\000\040\010\000\200\041\044\015\000\002"},
 	{270, "expr_stmt", 0, 6, states_14,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
 	{271, "augassign", 0, 2, states_15,
@@ -1835,7 +1875,7 @@
 	{274, "pass_stmt", 0, 2, states_18,
 	 "\000\000\000\000\000\000\000\100\000\000\000\000\000\000\000\000\000\000\000\000\000\000"},
 	{275, "flow_stmt", 0, 2, states_19,
-	 "\000\000\000\000\000\000\000\000\360\000\000\000\000\000\000\000\000\000\000\000\000\001"},
+	 "\000\000\000\000\000\000\000\000\360\000\000\000\000\000\000\000\000\000\000\000\000\002"},
 	{276, "break_stmt", 0, 2, states_20,
 	 "\000\000\000\000\000\000\000\000\020\000\000\000\000\000\000\000\000\000\000\000\000\000"},
 	{277, "continue_stmt", 0, 2, states_21,
@@ -1843,7 +1883,7 @@
 	{278, "return_stmt", 0, 3, states_22,
 	 "\000\000\000\000\000\000\000\000\100\000\000\000\000\000\000\000\000\000\000\000\000\000"},
 	{279, "yield_stmt", 0, 2, states_23,
-	 "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\001"},
+	 "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\002"},
 	{280, "raise_stmt", 0, 7, states_24,
 	 "\000\000\000\000\000\000\000\000\200\000\000\000\000\000\000\000\000\000\000\000\000\000"},
 	{281, "import_stmt", 0, 2, states_25,
@@ -1869,7 +1909,7 @@
 	{291, "assert_stmt", 0, 5, states_35,
 	 "\000\000\000\000\000\000\000\000\000\000\100\000\000\000\000\000\000\000\000\000\000\000"},
 	{292, "compound_stmt", 0, 2, states_36,
-	 "\000\010\020\000\000\000\000\000\000\000\000\220\023\000\000\000\000\000\000\000\002\000"},
+	 "\000\010\020\000\000\000\000\000\000\000\000\220\023\000\000\000\000\000\000\000\004\000"},
 	{293, "if_stmt", 0, 8, states_37,
 	 "\000\000\000\000\000\000\000\000\000\000\000\020\000\000\000\000\000\000\000\000\000\000"},
 	{294, "while_stmt", 0, 8, states_38,
@@ -1885,7 +1925,7 @@
 	{299, "except_clause", 0, 5, states_43,
 	 "\000\000\000\000\000\000\000\000\000\000\000\000\100\000\000\000\000\000\000\000\000\000"},
 	{300, "suite", 0, 5, states_44,
-	 "\004\040\040\000\000\000\000\124\360\024\114\000\000\040\010\000\200\041\044\015\000\001"},
+	 "\004\040\040\000\000\000\000\124\360\024\114\000\000\040\010\000\200\041\044\015\000\002"},
 	{301, "testlist_safe", 0, 5, states_45,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
 	{302, "old_test", 0, 2, states_46,
@@ -1942,32 +1982,34 @@
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
 	{328, "dictmaker", 0, 5, states_72,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
-	{329, "classdef", 0, 8, states_73,
-	 "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\002\000"},
-	{330, "arglist", 0, 8, states_74,
+	{329, "dictorsetmaker", 0, 8, states_73,
+	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
+	{330, "classdef", 0, 8, states_74,
+	 "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\004\000"},
+	{331, "arglist", 0, 8, states_75,
 	 "\000\040\040\300\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
-	{331, "argument", 0, 4, states_75,
+	{332, "argument", 0, 4, states_76,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
-	{332, "list_iter", 0, 2, states_76,
+	{333, "list_iter", 0, 2, states_77,
 	 "\000\000\000\000\000\000\000\000\000\000\000\020\001\000\000\000\000\000\000\000\000\000"},
-	{333, "list_for", 0, 6, states_77,
+	{334, "list_for", 0, 6, states_78,
 	 "\000\000\000\000\000\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000\000\000"},
-	{334, "list_if", 0, 4, states_78,
+	{335, "list_if", 0, 4, states_79,
 	 "\000\000\000\000\000\000\000\000\000\000\000\020\000\000\000\000\000\000\000\000\000\000"},
-	{335, "gen_iter", 0, 2, states_79,
+	{336, "gen_iter", 0, 2, states_80,
 	 "\000\000\000\000\000\000\000\000\000\000\000\020\001\000\000\000\000\000\000\000\000\000"},
-	{336, "gen_for", 0, 6, states_80,
+	{337, "gen_for", 0, 6, states_81,
 	 "\000\000\000\000\000\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000\000\000"},
-	{337, "gen_if", 0, 4, states_81,
+	{338, "gen_if", 0, 4, states_82,
 	 "\000\000\000\000\000\000\000\000\000\000\000\020\000\000\000\000\000\000\000\000\000\000"},
-	{338, "testlist1", 0, 2, states_82,
+	{339, "testlist1", 0, 2, states_83,
 	 "\000\040\040\000\000\000\000\000\000\000\000\000\000\040\010\000\200\041\044\015\000\000"},
-	{339, "encoding_decl", 0, 2, states_83,
+	{340, "encoding_decl", 0, 2, states_84,
 	 "\000\000\040\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"},
-	{340, "yield_expr", 0, 3, states_84,
-	 "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\001"},
+	{341, "yield_expr", 0, 3, states_85,
+	 "\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\002"},
 };
-static label labels[169] = {
+static label labels[170] = {
 	{0, "EMPTY"},
 	{256, 0},
 	{4, 0},
@@ -1982,11 +2024,11 @@
 	{50, 0},
 	{288, 0},
 	{7, 0},
-	{330, 0},
+	{331, 0},
 	{8, 0},
 	{260, 0},
 	{261, 0},
-	{329, 0},
+	{330, 0},
 	{262, 0},
 	{1, "def"},
 	{1, 0},
@@ -2013,7 +2055,7 @@
 	{290, 0},
 	{291, 0},
 	{271, 0},
-	{340, 0},
+	{341, 0},
 	{37, 0},
 	{38, 0},
 	{39, 0},
@@ -2118,29 +2160,30 @@
 	{319, 0},
 	{10, 0},
 	{26, 0},
-	{328, 0},
+	{329, 0},
 	{27, 0},
 	{25, 0},
-	{338, 0},
+	{339, 0},
 	{2, 0},
 	{3, 0},
-	{333, 0},
-	{336, 0},
+	{334, 0},
+	{337, 0},
 	{323, 0},
 	{324, 0},
 	{325, 0},
+	{328, 0},
 	{1, "class"},
-	{331, 0},
 	{332, 0},
-	{334, 0},
+	{333, 0},
 	{335, 0},
-	{337, 0},
-	{339, 0},
+	{336, 0},
+	{338, 0},
+	{340, 0},
 	{1, "yield"},
 };
 grammar _PyParser_Grammar = {
-	85,
+	86,
 	dfas,
-	{169, labels},
+	{170, labels},
 	256
 };
Index: Python/ast.c
===================================================================
--- Python/ast.c	(revision 74298)
+++ Python/ast.c	(working copy)
@@ -1368,36 +1368,59 @@
         else
             return ast_for_listcomp(c, ch);
     case LBRACE: {
-        /* dictmaker: test ':' test (',' test ':' test)* [','] */
+        /* dictorsetmaker: test ':' test (',' test ':' test)* [','] |
+         *                 test (',' test)* [','])
+         */
         int i, size;
         asdl_seq *keys, *values;
-        
+
         ch = CHILD(n, 1);
-        size = (NCH(ch) + 1) / 4; /* +1 in case no trailing comma */
-        keys = asdl_seq_new(size, c->c_arena);
-        if (!keys)
-            return NULL;
-        
-        values = asdl_seq_new(size, c->c_arena);
-        if (!values)
-            return NULL;
-        
-        for (i = 0; i < NCH(ch); i += 4) {
-            expr_ty expression;
+        if (TYPE(ch) == RBRACE) {
+            /* it's an empty dict */
+            return Dict(NULL, NULL, LINENO(n), n->n_col_offset, c->c_arena);
+        } else if (NCH(ch) == 1 || TYPE(CHILD(ch, 1)) == COMMA) {
+            /* it's a simple set */
+            asdl_seq *elts;
+            size = (NCH(ch) + 1) / 2; /* +1 in case no trailing comma */
+            elts = asdl_seq_new(size, c->c_arena);
+            if (!elts)
+                return NULL;
+            for (i = 0; i < NCH(ch); i += 2) {
+                expr_ty expression;
+                expression = ast_for_expr(c, CHILD(ch, i));
+                if (!expression)
+                    return NULL;
+                asdl_seq_SET(elts, i / 2, expression);
+            }
+            return Set(elts, LINENO(n), n->n_col_offset, c->c_arena);
+        } else {
+            /* it's a dict */
+            size = (NCH(ch) + 1) / 4; /* +1 in case no trailing comma */
+            keys = asdl_seq_new(size, c->c_arena);
+            if (!keys)
+                return NULL;
             
-            expression = ast_for_expr(c, CHILD(ch, i));
-            if (!expression)
+            values = asdl_seq_new(size, c->c_arena);
+            if (!values)
                 return NULL;
+            
+            for (i = 0; i < NCH(ch); i += 4) {
+                expr_ty expression;
+                
+                expression = ast_for_expr(c, CHILD(ch, i));
+                if (!expression)
+                    return NULL;
 
-            asdl_seq_SET(keys, i / 4, expression);
+                asdl_seq_SET(keys, i / 4, expression);
 
-            expression = ast_for_expr(c, CHILD(ch, i + 2));
-            if (!expression)
-                return NULL;
+                expression = ast_for_expr(c, CHILD(ch, i + 2));
+                if (!expression)
+                    return NULL;
 
-            asdl_seq_SET(values, i / 4, expression);
+                asdl_seq_SET(values, i / 4, expression);
+            }
+            return Dict(keys, values, LINENO(n), n->n_col_offset, c->c_arena);
         }
-        return Dict(keys, values, LINENO(n), n->n_col_offset, c->c_arena);
     }
     case BACKQUOTE: { /* repr */
         expr_ty expression;
Index: Python/import.c
===================================================================
--- Python/import.c	(revision 74298)
+++ Python/import.c	(working copy)
@@ -75,9 +75,10 @@
        Python 2.7a0: 62181 (optimize conditional branches:
 			    introduce POP_JUMP_IF_FALSE and POP_JUMP_IF_TRUE)
        Python 2.7a0  62191 (introduce SETUP_WITH)
+       Python 2.7a0  62201 (introduce BUILD_SET)
 .
 */
-#define MAGIC (62191 | ((long)'\r'<<16) | ((long)'\n'<<24))
+#define MAGIC (62201 | ((long)'\r'<<16) | ((long)'\n'<<24))
 
 /* Magic word as global; note that _PyImport_Init() can change the
    value of this global to accommodate for alterations of how the
Index: Python/symtable.c
===================================================================
--- Python/symtable.c	(revision 74298)
+++ Python/symtable.c	(working copy)
@@ -1212,6 +1212,9 @@
 		VISIT_SEQ(st, expr, e->v.Dict.keys);
 		VISIT_SEQ(st, expr, e->v.Dict.values);
 		break;
+        case Set_kind:
+		VISIT_SEQ(st, expr, e->v.Set.elts);
+		break;
         case ListComp_kind:
 		VISIT(st, expr, e->v.ListComp.elt);
 		VISIT_SEQ(st, comprehension, e->v.ListComp.generators);
Index: Python/compile.c
===================================================================
--- Python/compile.c	(revision 74298)
+++ Python/compile.c	(working copy)
@@ -807,6 +807,7 @@
 			return 1;
 		case BUILD_TUPLE:
 		case BUILD_LIST:
+		case BUILD_SET:
 			return 1-oparg;
 		case BUILD_MAP:
 			return 1;
@@ -2902,6 +2903,11 @@
 			ADDOP(c, STORE_MAP);
 		}
 		break;
+	case Set_kind:
+		n = asdl_seq_LEN(e->v.Set.elts);
+		VISIT_SEQ(c, expr, e->v.Set.elts);
+		ADDOP_I(c, BUILD_SET, n);
+		break;
 	case ListComp_kind:
 		return compiler_listcomp(c, e);
 	case GeneratorExp_kind:
Index: Python/Python-ast.c
===================================================================
--- Python/Python-ast.c	(revision 74298)
+++ Python/Python-ast.c	(working copy)
@@ -188,6 +188,10 @@
         "keys",
         "values",
 };
+static PyTypeObject *Set_type;
+static char *Set_fields[]={
+        "elts",
+};
 static PyTypeObject *ListComp_type;
 static char *ListComp_fields[]={
         "elt",
@@ -718,6 +722,8 @@
         if (!IfExp_type) return 0;
         Dict_type = make_type("Dict", expr_type, Dict_fields, 2);
         if (!Dict_type) return 0;
+        Set_type = make_type("Set", expr_type, Set_fields, 1);
+        if (!Set_type) return 0;
         ListComp_type = make_type("ListComp", expr_type, ListComp_fields, 2);
         if (!ListComp_type) return 0;
         GeneratorExp_type = make_type("GeneratorExp", expr_type,
@@ -1590,6 +1596,20 @@
 }
 
 expr_ty
+Set(asdl_seq * elts, int lineno, int col_offset, PyArena *arena)
+{
+        expr_ty p;
+        p = (expr_ty)PyArena_Malloc(arena, sizeof(*p));
+        if (!p)
+                return NULL;
+        p->kind = Set_kind;
+        p->v.Set.elts = elts;
+        p->lineno = lineno;
+        p->col_offset = col_offset;
+        return p;
+}
+
+expr_ty
 ListComp(expr_ty elt, asdl_seq * generators, int lineno, int col_offset,
          PyArena *arena)
 {
@@ -2566,6 +2586,15 @@
                         goto failed;
                 Py_DECREF(value);
                 break;
+        case Set_kind:
+                result = PyType_GenericNew(Set_type, NULL, NULL);
+                if (!result) goto failed;
+                value = ast2obj_list(o->v.Set.elts, ast2obj_expr);
+                if (!value) goto failed;
+                if (PyObject_SetAttrString(result, "elts", value) == -1)
+                        goto failed;
+                Py_DECREF(value);
+                break;
         case ListComp_kind:
                 result = PyType_GenericNew(ListComp_type, NULL, NULL);
                 if (!result) goto failed;
@@ -4725,6 +4754,38 @@
                 if (*out == NULL) goto failed;
                 return 0;
         }
+        if (PyObject_IsInstance(obj, (PyObject*)Set_type)) {
+                asdl_seq* elts;
+
+                if (PyObject_HasAttrString(obj, "elts")) {
+                        int res;
+                        Py_ssize_t len;
+                        Py_ssize_t i;
+                        tmp = PyObject_GetAttrString(obj, "elts");
+                        if (tmp == NULL) goto failed;
+                        if (!PyList_Check(tmp)) {
+                                PyErr_Format(PyExc_TypeError, "Set field \"elts\" must be a list, not a %.200s", tmp->ob_type->tp_name);
+                                goto failed;
+                        }
+                        len = PyList_GET_SIZE(tmp);
+                        elts = asdl_seq_new(len, arena);
+                        if (elts == NULL) goto failed;
+                        for (i = 0; i < len; i++) {
+                                expr_ty value;
+                                res = obj2ast_expr(PyList_GET_ITEM(tmp, i), &value, arena);
+                                if (res != 0) goto failed;
+                                asdl_seq_SET(elts, i, value);
+                        }
+                        Py_XDECREF(tmp);
+                        tmp = NULL;
+                } else {
+                        PyErr_SetString(PyExc_TypeError, "required field \"elts\" missing from Set");
+                        return 1;
+                }
+                *out = Set(elts, lineno, col_offset, arena);
+                if (*out == NULL) goto failed;
+                return 0;
+        }
         if (PyObject_IsInstance(obj, (PyObject*)ListComp_type)) {
                 expr_ty elt;
                 asdl_seq* generators;
@@ -6001,6 +6062,7 @@
             return;
         if (PyDict_SetItemString(d, "IfExp", (PyObject*)IfExp_type) < 0) return;
         if (PyDict_SetItemString(d, "Dict", (PyObject*)Dict_type) < 0) return;
+        if (PyDict_SetItemString(d, "Set", (PyObject*)Set_type) < 0) return;
         if (PyDict_SetItemString(d, "ListComp", (PyObject*)ListComp_type) < 0)
             return;
         if (PyDict_SetItemString(d, "GeneratorExp",
Index: Include/opcode.h
===================================================================
--- Include/opcode.h	(revision 74298)
+++ Include/opcode.h	(working copy)
@@ -99,13 +99,14 @@
 #define LOAD_NAME	101	/* Index in name list */
 #define BUILD_TUPLE	102	/* Number of tuple items */
 #define BUILD_LIST	103	/* Number of list items */
-#define BUILD_MAP	104	/* Always zero for now */
-#define LOAD_ATTR	105	/* Index in name list */
-#define COMPARE_OP	106	/* Comparison operator */
-#define IMPORT_NAME	107	/* Index in name list */
-#define IMPORT_FROM	108	/* Index in name list */
+#define BUILD_SET	104     /* Number of set items */
+#define BUILD_MAP	105	/* Always zero for now */
+#define LOAD_ATTR	106	/* Index in name list */
+#define COMPARE_OP	107	/* Comparison operator */
+#define IMPORT_NAME	108	/* Index in name list */
+#define IMPORT_FROM	109	/* Index in name list */
+#define JUMP_FORWARD	110	/* Number of bytes to skip */
 
-#define JUMP_FORWARD	110	/* Number of bytes to skip */
 #define JUMP_IF_FALSE_OR_POP 111 /* Target byte offset from beginning
                                     of code */
 #define JUMP_IF_TRUE_OR_POP 112	/* "" */
Index: Include/graminit.h
===================================================================
--- Include/graminit.h	(revision 74298)
+++ Include/graminit.h	(working copy)
@@ -73,15 +73,16 @@
 #define exprlist 326
 #define testlist 327
 #define dictmaker 328
-#define classdef 329
-#define arglist 330
-#define argument 331
-#define list_iter 332
-#define list_for 333
-#define list_if 334
-#define gen_iter 335
-#define gen_for 336
-#define gen_if 337
-#define testlist1 338
-#define encoding_decl 339
-#define yield_expr 340
+#define dictorsetmaker 329
+#define classdef 330
+#define arglist 331
+#define argument 332
+#define list_iter 333
+#define list_for 334
+#define list_if 335
+#define gen_iter 336
+#define gen_for 337
+#define gen_if 338
+#define testlist1 339
+#define encoding_decl 340
+#define yield_expr 341
Index: Include/Python-ast.h
===================================================================
--- Include/Python-ast.h	(revision 74298)
+++ Include/Python-ast.h	(working copy)
@@ -185,11 +185,11 @@
 };
 
 enum _expr_kind {BoolOp_kind=1, BinOp_kind=2, UnaryOp_kind=3, Lambda_kind=4,
-                  IfExp_kind=5, Dict_kind=6, ListComp_kind=7,
-                  GeneratorExp_kind=8, Yield_kind=9, Compare_kind=10,
-                  Call_kind=11, Repr_kind=12, Num_kind=13, Str_kind=14,
-                  Attribute_kind=15, Subscript_kind=16, Name_kind=17,
-                  List_kind=18, Tuple_kind=19};
+                  IfExp_kind=5, Dict_kind=6, Set_kind=7, ListComp_kind=8,
+                  GeneratorExp_kind=9, Yield_kind=10, Compare_kind=11,
+                  Call_kind=12, Repr_kind=13, Num_kind=14, Str_kind=15,
+                  Attribute_kind=16, Subscript_kind=17, Name_kind=18,
+                  List_kind=19, Tuple_kind=20};
 struct _expr {
         enum _expr_kind kind;
         union {
@@ -226,6 +226,10 @@
                 } Dict;
                 
                 struct {
+                        asdl_seq *elts;
+                } Set;
+                
+                struct {
                         expr_ty elt;
                         asdl_seq *generators;
                 } ListComp;
@@ -449,6 +453,8 @@
 #define Dict(a0, a1, a2, a3, a4) _Py_Dict(a0, a1, a2, a3, a4)
 expr_ty _Py_Dict(asdl_seq * keys, asdl_seq * values, int lineno, int
                  col_offset, PyArena *arena);
+#define Set(a0, a1, a2, a3) _Py_Set(a0, a1, a2, a3)
+expr_ty _Py_Set(asdl_seq * elts, int lineno, int col_offset, PyArena *arena);
 #define ListComp(a0, a1, a2, a3, a4) _Py_ListComp(a0, a1, a2, a3, a4)
 expr_ty _Py_ListComp(expr_ty elt, asdl_seq * generators, int lineno, int
                      col_offset, PyArena *arena);
Index: Grammar/Grammar
===================================================================
--- Grammar/Grammar	(revision 74298)
+++ Grammar/Grammar	(working copy)
@@ -114,7 +114,7 @@
 power: atom trailer* ['**' factor]
 atom: ('(' [yield_expr|testlist_gexp] ')' |
        '[' [listmaker] ']' |
-       '{' [dictmaker] '}' |
+       '{' [dictorsetmaker] '}' |
        '`' testlist1 '`' |
        NAME | NUMBER | STRING+)
 listmaker: test ( list_for | (',' test)* [','] )
@@ -127,6 +127,8 @@
 exprlist: expr (',' expr)* [',']
 testlist: test (',' test)* [',']
 dictmaker: test ':' test (',' test ':' test)* [',']
+dictorsetmaker: ( (test ':' test (',' test ':' test)* [',']) |
+                  (test (',' test)* [',']) )
 
 classdef: 'class' NAME ['(' [testlist] ')'] ':' suite
 
Index: Parser/Python.asdl
===================================================================
--- Parser/Python.asdl	(revision 74298)
+++ Parser/Python.asdl	(working copy)
@@ -56,6 +56,7 @@
 	     | Lambda(arguments args, expr body)
 	     | IfExp(expr test, expr body, expr orelse)
 	     | Dict(expr* keys, expr* values)
+	     | Set(expr* elts)
 	     | ListComp(expr elt, comprehension* generators)
 	     | GeneratorExp(expr elt, comprehension* generators)
 	     -- the grammar constrains where yield expressions can occur
Index: Doc/reference/expressions.rst
===================================================================
--- Doc/reference/expressions.rst	(revision 74298)
+++ Doc/reference/expressions.rst	(working copy)
@@ -65,7 +65,7 @@
 .. productionlist::
    atom: `identifier` | `literal` | `enclosure`
    enclosure: `parenth_form` | `list_display`
-            : | `generator_expression` | `dict_display`
+            : | `generator_expression` | `dict_display` | `set_display`
             : | `string_conversion` | `yield_atom`
 
 
@@ -281,6 +281,30 @@
 prevails.
 
 
+.. _set:
+
+Set displays
+------------
+
+.. index:: pair: set; display
+           object: set
+
+A set display is denoted by curly braces and distinguishable from dictionary
+displays by the lack of colons separating keys and values:
+
+.. productionlist::
+   set_display: "{" (`expression_list` | `comprehension`) "}"
+
+A set display yields a new mutable set object, the contents being specified by
+either a sequence of expressions or a comprehension.  When a comma-separated
+list of expressions is supplied, its elements are evaluated from left to right
+and added to the set object.  When a comprehension is supplied, the set is
+constructed from the elements resulting from the comprehension.
+
+An empty set cannot be constructed with ``{}``; this literal constructs an empty
+dictionary.
+
+
 .. _string-conversions:
 
 String conversions
Index: Lib/symbol.py
===================================================================
--- Lib/symbol.py	(revision 74298)
+++ Lib/symbol.py	(working copy)
@@ -83,18 +83,19 @@
 exprlist = 326
 testlist = 327
 dictmaker = 328
-classdef = 329
-arglist = 330
-argument = 331
-list_iter = 332
-list_for = 333
-list_if = 334
-gen_iter = 335
-gen_for = 336
-gen_if = 337
-testlist1 = 338
-encoding_decl = 339
-yield_expr = 340
+dictorsetmaker = 329
+classdef = 330
+arglist = 331
+argument = 332
+list_iter = 333
+list_for = 334
+list_if = 335
+gen_iter = 336
+gen_for = 337
+gen_if = 338
+testlist1 = 339
+encoding_decl = 340
+yield_expr = 341
 #--end constants--
 
 sym_name = {}
Index: Lib/compiler/pycodegen.py
===================================================================
--- Lib/compiler/pycodegen.py	(revision 74298)
+++ Lib/compiler/pycodegen.py	(working copy)
@@ -1215,6 +1215,12 @@
             self.visit(elt)
         self.emit('BUILD_LIST', len(node.nodes))
 
+    def visitSet(self, node):
+        self.set_lineno(node)
+        for elt in node.nodes:
+            self.visit(elt)
+        self.emit('BUILD_SET', len(node.nodes))
+
     def visitSliceobj(self, node):
         for child in node.nodes:
             self.visit(child)
Index: Lib/compiler/pyassem.py
===================================================================
--- Lib/compiler/pyassem.py	(revision 74298)
+++ Lib/compiler/pyassem.py	(working copy)
@@ -734,6 +734,8 @@
         return -count+1
     def BUILD_LIST(self, count):
         return -count+1
+    def BUILD_SET(self, count):
+        return -count+1
     def CALL_FUNCTION(self, argc):
         hi, lo = divmod(argc, 256)
         return -(lo + hi * 2)
Index: Lib/compiler/transformer.py
===================================================================
--- Lib/compiler/transformer.py	(revision 74298)
+++ Lib/compiler/transformer.py	(working copy)
@@ -749,7 +749,7 @@
     def atom_lbrace(self, nodelist):
         if nodelist[1][0] == token.RBRACE:
             return Dict((), lineno=nodelist[0][2])
-        return self.com_dictmaker(nodelist[1])
+        return self.com_dictorsetmaker(nodelist[1])
 
     def atom_backquote(self, nodelist):
         return Backquote(self.com_node(nodelist[1]))
@@ -1197,13 +1197,21 @@
             assert node[0] == symbol.gen_iter
             return node[1]
 
-    def com_dictmaker(self, nodelist):
-        # dictmaker: test ':' test (',' test ':' value)* [',']
-        items = []
-        for i in range(1, len(nodelist), 4):
-            items.append((self.com_node(nodelist[i]),
-                          self.com_node(nodelist[i+2])))
-        return Dict(items, lineno=items[0][0].lineno)
+    def com_dictorsetmaker(self, nodelist):
+        # dictorsetmaker: ( (test ':' test (',' test ':' test)* [',']) |
+        #                   (test (',' test)* [',']) )
+        assert nodelist[0] == symbol.dictorsetmaker
+        if len(nodelist) == 2 or nodelist[2] == token.COMMA:
+            items = []
+            for i in range(1, len(nodelist), 2):
+                items.append(self.com_node(nodelist[i]))
+            return Set(items, lineno=items[0].lineno)
+        else:
+            items = []
+            for i in range(1, len(nodelist), 4):
+                items.append((self.com_node(nodelist[i]),
+                              self.com_node(nodelist[i+2])))
+            return Dict(items, lineno=items[0][0].lineno)
 
     def com_apply_trailer(self, primaryNode, nodelist):
         t = nodelist[1][0]
Index: Lib/compiler/ast.py
===================================================================
--- Lib/compiler/ast.py	(revision 74298)
+++ Lib/compiler/ast.py	(working copy)
@@ -1109,6 +1109,22 @@
     def __repr__(self):
         return "RightShift((%s, %s))" % (repr(self.left), repr(self.right))
 
+class Set(Node):
+    def __init__(self, nodes, lineno=None):
+        self.nodes = nodes
+        self.lineno = lineno
+
+    def getChildren(self):
+        return tuple(flatten(self.nodes))
+
+    def getChildNodes(self):
+        nodelist = []
+        nodelist.extend(flatten_nodes(self.nodes))
+        return tuple(nodelist)
+
+    def __repr__(self):
+        return "Set(%s)" % (repr(self.nodes),)
+
 class Slice(Node):
     def __init__(self, expr, flags, lower, upper, lineno=None):
         self.expr = expr
Index: Lib/opcode.py
===================================================================
--- Lib/opcode.py	(revision 74298)
+++ Lib/opcode.py	(working copy)
@@ -138,13 +138,13 @@
 name_op('LOAD_NAME', 101)       # Index in name list
 def_op('BUILD_TUPLE', 102)      # Number of tuple items
 def_op('BUILD_LIST', 103)       # Number of list items
-def_op('BUILD_MAP', 104)        # Number of dict entries (upto 255)
-name_op('LOAD_ATTR', 105)       # Index in name list
-def_op('COMPARE_OP', 106)       # Comparison operator
-hascompare.append(106)
-name_op('IMPORT_NAME', 107)     # Index in name list
-name_op('IMPORT_FROM', 108)     # Index in name list
-
+def_op('BUILD_SET', 104)        # Number of set items
+def_op('BUILD_MAP', 105)        # Number of dict entries (upto 255)
+name_op('LOAD_ATTR', 106)       # Index in name list
+def_op('COMPARE_OP', 107)       # Comparison operator
+hascompare.append(107)
+name_op('IMPORT_NAME', 108)     # Index in name list
+name_op('IMPORT_FROM', 109)     # Index in name list
 jrel_op('JUMP_FORWARD', 110)    # Number of bytes to skip
 jabs_op('JUMP_IF_FALSE_OR_POP', 111) # Target byte offset from beginning of code
 jabs_op('JUMP_IF_TRUE_OR_POP', 112)  # ""
Index: Lib/test/test_parser.py
===================================================================
--- Lib/test/test_parser.py	(revision 74298)
+++ Lib/test/test_parser.py	(working copy)
@@ -59,7 +59,20 @@
 
     def test_expressions(self):
         self.check_expr("foo(1)")
+        self.check_expr("{1:1}")
+        self.check_expr("{1:1, 2:2, 3:3}")
+        self.check_expr("{1:1, 2:2, 3:3,}")
+        self.check_expr("{1}")
+        self.check_expr("{1, 2, 3}")
+        self.check_expr("{1, 2, 3,}")
+        self.check_expr("[]")
+        self.check_expr("[1]")
         self.check_expr("[1, 2, 3]")
+        self.check_expr("[1, 2, 3,]")
+        self.check_expr("()")
+        self.check_expr("(1,)")
+        self.check_expr("(1, 2, 3)")
+        self.check_expr("(1, 2, 3,)")
         self.check_expr("[x**3 for x in range(20)]")
         self.check_expr("[x**3 for x in range(20) if x % 3]")
         self.check_expr("[x**3 for x in range(20) if x % 2 if x % 3]")
Index: Lib/test/test_compiler.py
===================================================================
--- Lib/test/test_compiler.py	(revision 74298)
+++ Lib/test/test_compiler.py	(working copy)
@@ -248,6 +248,7 @@
 l[3:4]
 d = {'a': 2}
 d = {}
+s = {1}
 t = ()
 t = (1, 2)
 l = []
Index: Lib/test/test_grammar.py
===================================================================
--- Lib/test/test_grammar.py	(revision 74298)
+++ Lib/test/test_grammar.py	(working copy)
@@ -749,7 +749,7 @@
 
     def testAtoms(self):
         ### atom: '(' [testlist] ')' | '[' [testlist] ']' | '{' [dictmaker] '}' | '`' testlist '`' | NAME | NUMBER | STRING
-        ### dictmaker: test ':' test (',' test ':' test)* [',']
+        ### dictorsetmaker: (test ':' test (',' test ':' test)* [',']) | (test (',' test)* [','])
 
         x = (1)
         x = (1 or 2 or 3)
@@ -769,6 +769,11 @@
         x = {'one': 1, 'two': 2,}
         x = {'one': 1, 'two': 2, 'three': 3, 'four': 4, 'five': 5, 'six': 6}
 
+        x = {'one'}
+        x = {'one', 1,}
+        x = {'one', 'two', 'three'}
+        x = {2, 3, 4,}
+
         x = `x`
         x = `1 or 2 or 3`
         self.assertEqual(`1,2`, '(1, 2)')
Index: Modules/parsermodule.c
===================================================================
--- Modules/parsermodule.c	(revision 74298)
+++ Modules/parsermodule.c	(working copy)
@@ -935,7 +935,7 @@
 VALIDATER(atom);                VALIDATER(lambdef);
 VALIDATER(trailer);             VALIDATER(subscript);
 VALIDATER(subscriptlist);       VALIDATER(sliceop);
-VALIDATER(exprlist);            VALIDATER(dictmaker);
+VALIDATER(exprlist);            VALIDATER(dictorsetmaker);
 VALIDATER(arglist);             VALIDATER(argument);
 VALIDATER(listmaker);           VALIDATER(yield_stmt);
 VALIDATER(testlist1);           VALIDATER(gen_for);
@@ -2478,7 +2478,7 @@
                    && validate_ntype(CHILD(tree, nch - 1), RBRACE));
 
             if (res && (nch == 3))
-                res = validate_dictmaker(CHILD(tree, 1));
+                res = validate_dictorsetmaker(CHILD(tree, 1));
             break;
           case BACKQUOTE:
             res = ((nch == 3)
@@ -2966,32 +2966,59 @@
 
 
 static int
-validate_dictmaker(node *tree)
+validate_dictorsetmaker(node *tree)
 {
     int nch = NCH(tree);
-    int res = (validate_ntype(tree, dictmaker)
-               && (nch >= 3)
-               && validate_test(CHILD(tree, 0))
-               && validate_colon(CHILD(tree, 1))
-               && validate_test(CHILD(tree, 2)));
+    int ok = validate_ntype(tree, dictorsetmaker);
+    int i = 0;
 
-    if (res && ((nch % 4) == 0))
-        res = validate_comma(CHILD(tree, --nch));
-    else if (res)
-        res = ((nch % 4) == 3);
+    assert(nch > 0);
 
-    if (res && (nch > 3)) {
-        int pos = 3;
-        /*  ( ',' test ':' test )*  */
-        while (res && (pos < nch)) {
-            res = (validate_comma(CHILD(tree, pos))
-                   && validate_test(CHILD(tree, pos + 1))
-                   && validate_colon(CHILD(tree, pos + 2))
-                   && validate_test(CHILD(tree, pos + 3)));
-            pos += 4;
+    if (ok && (nch == 1 || TYPE(CHILD(tree, 1)) == COMMA)) {
+        /* We got a set:
+         *     test (',' test)* [','] 
+         */
+        ok = validate_test(CHILD(tree, i++));
+        while (ok && nch - i >= 2) {
+            ok = (validate_comma(CHILD(tree, i))
+                   && validate_test(CHILD(tree, i+1)));
+            i += 2;
         }
     }
-    return (res);
+    else if (ok) {
+        /* We got a dict:
+         *     test ':' test (',' test ':' test)* [',']
+         */
+        if (nch >= 3) {
+            ok = (validate_test(CHILD(tree, i))
+                  && validate_colon(CHILD(tree, i+1))
+                  && validate_test(CHILD(tree, i+2)));
+            i += 3;
+        }
+        else {
+            ok = 0;
+            err_string("illegal number of nodes for dictorsetmaker");
+        }
+
+        while (ok && nch - i >= 4) {
+            ok = (validate_comma(CHILD(tree, i))
+                  && validate_test(CHILD(tree, i+1))
+                  && validate_colon(CHILD(tree, i+2))
+                  && validate_test(CHILD(tree, i+3)));
+            i += 4;
+        }
+    }
+    /* Check for a trailing comma. */
+    if (ok) {
+        if (i == nch-1)
+            ok = validate_comma(CHILD(tree, i));
+        else if (i != nch) {
+            ok = 0;
+            err_string("illegal trailing nodes for dictorsetmaker");
+        }
+    }
+
+    return ok;
 }