Experimental parser: add comments for NfaBuilder.

tools/lexer_generator/nfa_builder.py

Index: tools/lexer_generator/nfa_builder.py
diff --git a/tools/lexer_generator/nfa_builder.py b/tools/lexer_generator/nfa_builder.py
index 5ff7e242b90cd08b944c5ecacee3a57695be3159..af170e439c2bc2fa22b314e661c82bcf939fd5aa 100644
--- a/tools/lexer_generator/nfa_builder.py
+++ b/tools/lexer_generator/nfa_builder.py
@@ -29,6 +29,15 @@ from types import TupleType
 from inspect import getmembers
 from nfa import *
 
+# Nfa is built in two stages:
+# 1. Build a tree of operations on rules.
+#    Each node in the tree is a tuple (operation, subtree1, ... subtreeN).
+#    Rule parser builds this tree by invoking static methods of NfaBuilder.
+# 2. For each node, perform the operation of the node to produce an Nfa.
+#    If an operation is called X, then it is performed by the method
+#    of NfaBuilder called __x(). See __process() for mapping from
+#    operation to processing methods.
+
 class NfaBuilder(object):
 
   def __init__(self, encoding, character_classes = {}):