code: public function parse($formula)
      {
          $this->currentCharacter = 0;
          $this->formula = $formula;
          $this->lookAhead = isset($formula[1]) ? $formula[1]
          : '';
          $this->advance();
          $this->parseTree = $this->condition();
  
          return true;
      }
docs: The parsing method. It parses a formula.
  
  @param string $formula the formula to parse, without the initial equal
  sign (=)
  
  @return mixed true on success