diff -r 128b45caba5b compiler.rst
--- a/compiler.rst	Sun Jun 26 09:01:18 2016 +0300
+++ b/compiler.rst	Fri Jul 08 19:08:51 2016 +0100
@@ -74,6 +74,26 @@
 'else' statement.  ``REQ(CHILD(node, 2), COLON)`` can be used to access
 what should be the first ``:`` and require it be an actual ``:`` token.
 
+As an optimization, certain kinds of nodes (listed in the big switch
+statement in ``PyNode_Compress``), when they only have a single child,
+are replaced with the child node. This allows "compressing" long
+branchless stretches in the parse tree, significantly lowering the
+peak memory consumption by the parser: back in 2002, Andrew MacIntyre
+reported that up to 89% of nodes in a parse tree have a single child;
+the "node compression" allows most of these nodes to be eliminated.
+
+The implication for parse tree consumers is that a node's children
+may not have exactly the types as defined in Grammar/Grammar; any
+child node could have been replaced with a more specific subclass.
+For example, a ``testlist`` node can now have an ``atom`` node as a
+direct child, instead of a::
+
+  test -> or_test -> and_test -> not_test -> comparison ->
+       expr -> xor_expr -> and_expr -> shift_expr -> 
+       arith_expr -> term -> factor -> power -> atom_expr -> atom
+
+degenerate subtree of single-child nodes.
+
 
 Abstract Syntax Trees (AST)
 ---------------------------