public function add($key, $value) {
		$this->validateValue($value);
		if (!isset($this->variables[$key])) {
			$this->variables[$key] = [];
		}

		if (!is_array($this->variables[$key])) {
			$this->variables[$key] = [$this->variables[$key]];
		}

		$this->variables[$key][] = $value;
	}