Index: Lib/lib2to3/pytree.py
===================================================================
--- Lib/lib2to3/pytree.py	(revision 77840)
+++ Lib/lib2to3/pytree.py	(working copy)
@@ -132,6 +132,8 @@
 
     def replace(self, new):
         """Replace this node with a new one in the parent."""
+        if self is new:
+            return
         assert self.parent is not None, str(self)
         assert new is not None
         if not isinstance(new, list):
@@ -149,9 +151,12 @@
         assert found, (self.children, self, new)
         self.parent.changed()
         self.parent.children = l_children
+        # one of the new nodes may be self;
+        # make sure it doesn't lose its parent.
+        parent = self.parent
+        self.parent = None
         for x in new:
-            x.parent = self.parent
-        self.parent = None
+            x.parent = parent
 
     def get_lineno(self):
         """Return the line number which generated the invocant node."""
Index: Lib/lib2to3/tests/test_pytree.py
===================================================================
--- Lib/lib2to3/tests/test_pytree.py	(revision 77840)
+++ Lib/lib2to3/tests/test_pytree.py	(working copy)
@@ -157,6 +157,8 @@
         self.assertEqual(n1.children, [l1, l2, l3])
         self.assertTrue(isinstance(n1.children, list))
         self.assertFalse(n1.was_changed)
+        l2.replace(l2)
+        self.assertEqual(l2.parent, n1)
         l2new = pytree.Leaf(100, "-")
         l2.replace(l2new)
         self.assertEqual(n1.children, [l1, l2new, l3])
@@ -173,6 +175,22 @@
         self.assertEqual(str(n1), "foo**bar")
         self.assertTrue(isinstance(n1.children, list))
 
+        l1.replace([l1, l2, pytree.Leaf(100, "baz")])
+        self.assertEqual(l1.parent, n1)
+        self.assertEqual(str(n1), "foo+baz**bar")
+        self.assertTrue(isinstance(n1.children, list))
+
+        l2.replace([pytree.Leaf(100, "*"), pytree.Leaf(100, "bar"), l2])
+        self.assertEqual(l2.parent, n1)
+        self.assertEqual(str(n1), "foo*bar+baz**bar")
+        self.assertTrue(isinstance(n1.children, list))
+
+        l2.replace([pytree.Leaf(100, "*"), pytree.Leaf(100, "foo"), l2, 
+                    pytree.Leaf(100, "bar"), pytree.Leaf(100, "*")])
+        self.assertEqual(l2.parent, n1)
+        self.assertEqual(str(n1), "foo*bar*foo+bar*baz**bar")
+        self.assertTrue(isinstance(n1.children, list))
+
     def test_post_order(self):
         l1 = pytree.Leaf(100, "foo")
         l2 = pytree.Leaf(100, "bar")