code: void appendSubRules( String[] parentSelector, CssFormatter formatter ) {
          try {
              if( important ) {
                  formatter.incImportant();
              }
              for( MixinMatch match : getRules( formatter ) ) {
                  Rule rule = match.getRule();
                  formatter.addMixin( rule, match.getMixinParameters(), rule.getVariables() );
                  rule.appendMixinsTo( parentSelector, formatter );
                  for( Rule subMixin : rule.getSubrules() ) {
                      if( !subMixin.isMixin() && (parentSelector == null || !subMixin.isInlineRule( formatter ) ) ) {
                          subMixin.appendTo( parentSelector, formatter );
                      }
                  }
                  formatter.removeMixin();
              }
          } catch( LessException ex ) {
              ex.addPosition( filename, line, column );
              throw ex;
          } finally {
              if( important ) {
                  formatter.decImportant();
              }
          }
      }
docs: Append the rules of the mixins to the formatter.
  @param parentSelector the selectors of the caller
  @param formatter the formatter