anchor.diff:      for (var i = lists.length - 1; i >= 0; i--) {
         middle:
         for (var complex1 in lists[i].reversed) {
  -        if (_originals.contains(complex1)) {
  +        if (isOriginal(complex1)) {
             // Make sure we don't include duplicate originals, which could happen
             // if a style rule extends a component of its own selector.
             for (var j = 0; j < numOriginals; j++) {
  
positive.code:   /// Extends [compound] using [extensions], and returns the contents of a
    /// [SelectorList].
    ///
    /// If [replace] is `true`, this doesn't preserve the original selectors.
    List<ComplexSelector> _extendCompound(
        CompoundSelector compound,
        Map<SimpleSelector, Map<ComplexSelector, Extension>> extensions,
        List<CssMediaQuery> mediaQueryContext,
        {bool replace: false}) {
      // The complex selectors produced from each component of [compound].
      List<List<Extension>> options;
      for (var i = 0; i < compound.components.length; i++) {
  
negative.code:     }
      if (extended == null) return list;
  
      return new SelectorList(_trim(extended, _originals.contains)
          .where((complex) => complex != null));
    }
  
    /// Extends [complex] using [extensions], and returns the contents of a
  