code: public function isImplemented($pFunction)
      {
          $pFunction = strtoupper($pFunction);
          $notImplemented = !isset(self::$phpSpreadsheetFunctions[$pFunction]) || (is_array(self::$phpSpreadsheetFunctions[$pFunction]['functionCall']) && self::$phpSpreadsheetFunctions[$pFunction]['functionCall'][1] === 'DUMMY');
  
          return !$notImplemented;
      }
docs: Is a specific function implemented?
  
  @param string $pFunction Function Name
  
  @return bool