anchor.diff:    CssStyleRule addSelector(CssValue<SelectorList> selector, FileSpan span,
         [List<CssMediaQuery> mediaContext]) {
       for (var complex in selector.value.components) {
  -      _original[complex] = true;
  +      _originals.add(complex);
       }
   
       if (_extensions.isNotEmpty) {
  
positive.code:         // Make sure that copies of [complex] retain their status as "original"
          // selectors. This includes selectors that are modified because a :not()
          // was extended into.
          if (first && _original[complex] != null) {
            _original[outputComplex] = true;
          }
          first = false;
  
  
negative.code:   /// of extend][].
    ///
    /// [second law of extend]: https://github.com/sass/sass/issues/324#issuecomment-4607184
    final _sourceSpecificity = new Map<SimpleSelector, int>.identity();
  
    /// A set of [ComplexSelector]s that were originally part of
    /// their component [SelectorList]s, as opposed to being added by `@extend`.
    ///
    /// This allows us to ensure that we do'nt trim any selectors that need to
    /// exist to satisfy the [first law of extend][].
    ///
    /// [first law of extend]: https://github.com/sass/sass/issues/324#issuecomment-4607184
    final _originals = new Set<ComplexSelector>.identity();
  
    /// Extends [selector] with [source] extender and [target] the extendee.
    ///
  