code: private function expr($rbp = 0)
      {
          $left = $this->{"nud_{$this->token['type']}"}();
          while ($rbp < self::$bp[$this->token['type']]) {
              $left = $this->{"led_{$this->token['type']}"}($left);
          }
  
          return $left;
      }
docs: Parses an expression while rbp < lbp.
  
  @param int   $rbp  Right bound precedence
  
  @return array