anchor.diff:    /// The human-readable name of the current stack frame.
     var _member = "root stylesheet";
   
  +  /// The span for the innermost callable that's been invoked.
  +  ///
  +  /// This is used to provide `call()` with a span.
  +  FileSpan _callableSpan;
  +
     /// The resolved URLs for each [ImportRule] that's been seen so far.
     ///
     /// This is cached in case the same file is imported multiple times, and thus
  
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 args = arguments[1] as SassArgumentList;
  
        var expression = new FunctionExpression(
            new Interpolation([name.text], _callableSpan),
            new ArgumentInvocation([], {}, _callableSpan,
                rest: new ValueExpression(args)));
        return expression.accept(this);
      });
  