anchor.diff:        var args = arguments[1] as SassArgumentList;
   
         var expression = new FunctionExpression(
  -          new Interpolation([name.text], null),
  -          new ArgumentInvocation([], {}, null,
  +          new Interpolation([name.text], _callableSpan),
  +          new ArgumentInvocation([], {}, _callableSpan,
                 rest: new ValueExpression(args)));
         return expression.accept(this);
       });
  
positive.code:     var namedSet = named;
      var separator = triple.item3;
  
      int overloadIndex;
      for (var i = 0; i < callable.overloads.length - 1; i++) {
        try {
  
negative.code:     }
  
      var result = _addExceptionSpan(invocation.span, () => callback(positional));
      _callableSpan = oldCallableSpan;
  
      if (argumentList == null) return result;
      if (named.isEmpty) return result;
  