diff --git a/stl/ast.py b/stl/ast.py
index 6ed256a..af1c3a0 100644
--- a/stl/ast.py
+++ b/stl/ast.py
@@ -36,6 +36,9 @@ class AST(object):
     def __invert__(self):
         return Neg(self)
 
+    def children(self):
+        return []
+
 
 class _Top(AST):
     def __repr__(self):