code: private Operation range( CssFormatter formatter ) {
          type = LIST;
          double start = get( 0 ).doubleValue( formatter );
          double end;
          if( parameters.size() >= 2 ) {
              end = get( 1 ).doubleValue( formatter );
          } else {
              end = start;
              start = 1;
          }
          double step;
          if( parameters.size() >= 3 ) {
              step = get( 2 ).doubleValue( formatter );
          } else {
              step = 1;
          }
          String unit = get( 0 ).unit( formatter );
          Operation op = new Operation( this, ' ' );
          while( start <= end ) {
              op.addOperand( new ValueExpression( this, start + unit ) );
              start += step;
          }
          return op;
      }
docs: Function range. Generate a list spanning a range of values
  
  @param formatter
  current CSS output
  @return The operation representing a list of values