Parser

- Implemented ::scan, which iterates over parse tree nodes and calls back.
  This method skips parent nodes, terminated nodes, and nodes without the '?'
  tag.

src/Parser.h

@@ -69,6 +69,7 @@ public:
 
 private:
   void findTerminator ();
+  void scan (void (Parser::*callback)(Tree*));
   void findPattern ();
   void findSubstitution ();
   void findTag ();