code: void appendMixinsTo( String[] parentSelector, CssFormatter formatter ) {
          for( Formattable prop : properties ) {
              switch( prop.getType()) {
                  case MIXIN:
                      ((Mixin)prop).appendSubRules( parentSelector, formatter );
                      break;
                  case CSS_AT_RULE:
                  case COMMENT:
                      prop.appendTo( formatter );
                      break;
              }
          }
      }
docs: Append the mixins of this rule to current output.
  
  @param parentSelector the resulting parent selector
  @param formatter current formatter