add Node.visit() method

parser/nodes.py

@@ -4,6 +4,10 @@ class Node(object):
         self.children = children
         if self.children is None:
             self.children = []
+    def visit(self, callback, *args):
+        callback(self, *args)
+        for c in self.children:
+            c.visit(callback, *args)
 
 class StatementNode(Node):
     def __init__(self, children=None):