add python visitor example in response to Stackoverflow question

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

examples/python/visitor.py

@@ -0,0 +1,29 @@
+# Copyright (c) Microsoft Corporation 2015
+
+from z3 import *
+
+def visitor(e, seen):
+    if e in seen:
+        return
+    seen[e] = True
+    yield e
+    if is_app(e):
+        for ch in e.children():
+            for e in visitor(ch, seen):
+                yield e
+        return
+    if is_quantifier(e):
+        for e in visitor(e.body(), seen):
+            yield e
+        return
+
+x, y = Ints('x y')
+fml = x + x + y > 2 
+seen = {}
+for e in visitor(fml, seen):
+    if is_const(e) and e.decl().kind() == Z3_OP_UNINTERPRETED:
+        print "Variable", e
+    else:
+        print e
+    
+