anchor.diff:            selector, {target: new Set()..add(new ExtendSource(source, null))},
             replace: true);
   
  -  CssStyleRule addSelector(
  -      CssValue<SelectorList> selectorValue, FileSpan span) {
  -    var selector = selectorValue.value;
  -    for (var complex in selector.components) {
  +  CssStyleRule addSelector(CssValue<SelectorList> selector, FileSpan span) {
  +    for (var complex in selector.value.components) {
         for (var component in complex.components) {
           if (component is CompoundSelector) {
             for (var simple in component.components) {
  
positive.code:       }
      }
  
      return extended;
    }
  
    List<ComplexSelector> _trim(List<List<ComplexSelector>> lists) {
  
negative.code:   List<ComplexSelector> _extendCompound(CompoundSelector compound,
        Map<SimpleSelector, Set<ExtendSource>> extensions,
        {bool replace: false}) {
      var original = compound;
  
      List<ComplexSelector> extended;
      for (var i = 0; i < compound.components.length; i++) {
        var simple = compound.components[i];
  
        var extendedPseudo = simple is PseudoSelector && simple.selector != null
            ? _extendPseudo(simple, extensions, replace: replace)
            : null;
  
        if (extendedPseudo != null) {
          var simples = new List<SimpleSelector>(
              compound.components.length - 1 + extendedPseudo.length);
          simples.setRange(0, i, compound.components);
          simples.setRange(i, i + extendedPseudo.length, extendedPseudo);
          simples.setRange(
              i + extendedPseudo.length, simples.length, compound.components, i);
          original = new CompoundSelector(simples);
        }
  
        var sources = extensions[simple];
        if (sources == null) continue;
  
        var compoundWithoutSimple =
            new List<SimpleSelector>(compound.components.length - 1);
        compoundWithoutSimple.setRange(0, i, compound.components);
        compoundWithoutSimple.setRange(
            i, compound.components.length - 1, compound.components, i);
        for (var source in sources) {
          for (var complex in source.extender.components) {
            var extenderBase = complex.components.last as CompoundSelector;
  