code: @Override
      public String unit( CssFormatter formatter ) {
          switch( operator ) {
              case '*':
              case '/':
                  ArrayList<Unit> list = unit( formatter, null );
                  if( list != null ) {
                      Unit unit = list.get( 0 );
                      int useCount = unit.useCount();
                      for( int i=1; i<list.size(); i++ ){
                          if( list.get( i ).useCount() > useCount ) {
                              unit = list.get( i );
                          }
                      }
                      return unit.unit;
                  }
                  return "";
          }
          for( int i = 0; i < operands.size(); i++ ) {
              String unit = operands.get( i ).unit( formatter );
              if( !unit.isEmpty() ) {
                  return unit;
              }
          }
          return "";
      }
docs: {@inheritDoc}