code: List<Rule> getMixin( String name ) {
          ArrayList<Rule> rules = null;
          for( Rule rule : subrules ) {
              for( String sel : rule.selectors ) {
                  if( name.equals( sel ) ) {
                      if( rules == null ) {
                          rules = new ArrayList<>();
                      }
                      rules.add( rule );
                      break;
                  }
              }
          }
          return rules;
      }
docs: Get a nested mixin of this rule.
  
  @param name
  the name of the mixin
  @return the mixin or null