implement AST validation (#3863)

lib/utils.js

@@ -147,6 +147,9 @@ var List = (function() {
         }
         return top.concat(ret);
     }
+    List.is_op = function(val) {
+        return val === skip || val instanceof AtTop || val instanceof Last || val instanceof Splice;
+    };
     List.at_top = function(val) { return new AtTop(val); };
     List.splice = function(val) { return new Splice(val); };
     List.last = function(val) { return new Last(val); };