code: List<Rule> getMixin( String name ) {
          for( int i = state.stackIdx - 1; i >= 0; i-- ) {
              Rule mixin = state.stack.get( i ).mixin;
              if( mixin != null ) {
                  List<Rule> rules = mixin.getMixin( name );
                  if( rules != null ) {
                      for( int r = 0; r < rules.size(); r++ ) {
                          if( !containsRule( rules.get( r ) ) ) {
                              return rules;
                          }
                      }
                  }
              }
          }
          return null;
      }
docs: Get a nested mixin of a parent rule.
  
  @param name
  the name of the mixin
  @return the mixin or null