diff --git a/src/Parser.cpp b/src/Parser.cpp
index 78e384eed..a2099b9ff 100644
--- a/src/Parser.cpp
+++ b/src/Parser.cpp
@@ -270,33 +270,24 @@ bool Parser::canonicalize (
 }
 
 ////////////////////////////////////////////////////////////////////////////////
-// Experimental method to iterate over nodes, and callback, which is essentially
-// a co-routine implementation.
-void Parser::scan (void (Parser::*callback) (Tree*))
+// Recursively scan all nodes, depth first, skipping terminated nodes, and known
+// nodes, and cal the callback function for each node.
+void Parser::scan (void (Parser::*callback) (Tree*), Tree* tree /* = NULL */)
 {
+  if (tree == NULL)
+    tree = _tree;
+
   std::vector <Tree*>::iterator i;
-  for (i = _tree->_branches.begin (); i != _tree->_branches.end (); ++i)
+  for (i = tree->_branches.begin (); i != tree->_branches.end (); ++i)
   {
     if ((*i)->_branches.size ())
     {
-      std::vector <Tree*>::iterator b;
-      for (b = (*i)->_branches.begin (); b != (*i)->_branches.end (); ++b)
-      {
-        // Parser override operator.
-        if ((*b)->attribute ("raw") == "--")
-          break;
-
-        // Skip known args.
-        if (! (*b)->hasTag ("?"))
-          continue;
-
-        (this->*callback) (*b);
-      }
+      scan (callback, *i);
     }
     else
     {
       // Parser override operator.
-      if ((*i)->attribute ("raw") == "--")
+      if ((*i)->hasTag ("TERMINATOR"))
         break;
 
       // Skip known args.
diff --git a/src/Parser.h b/src/Parser.h
index 6232f6379..cd03809c0 100644
--- a/src/Parser.h
+++ b/src/Parser.h
@@ -69,7 +69,7 @@ public:
 
 private:
   void findTerminator ();
-  void scan (void (Parser::*callback)(Tree*));
+  void scan (void (Parser::*callback)(Tree*), Tree* tree = NULL);
   void findPattern ();
   void findSubstitution ();
   void findTag ();