code: public function multiple($value) {
          $value = self::sanitize($value);
          switch (substr($value, -1)) {
              case '+':
              case '*':
                  break;
              default:
                  $value .= '+';
                  break;
          }
          return $this->add($value);
      }
docs: Multiple
  
  Adds the multiple modifier at the end of your expression
  
  @access public
  @param  string $value Your expression
  @return VerbalExpressions