private function _addCondition($condition)
	{
		if($this->logicOperator === null && $this->invoked === false)
		{
			$this->conditionArray[] = array($condition);
			$this->invoked          = true;
		}
		elseif($this->logicOperator === null)
		{
			throw new \Exception('a logic operator method have to be invoked to continue add conditions');
		}
		else
		{
			$this->conditionArray[] = array($this->logicOperator => $condition);
			$this->logicOperator    = null;
		}
	}