public function add( $key, $value ) {
		if ( ! array_key_exists( $key, $this->attributes ) ) {
			$this->attributes[ $key ] = $value;
		} else {
			if ( ! \is_array( $this->attributes[ $key ] ) ) {
				$this->attributes[ $key ] = array( $this->attributes[ $key ], $value );
			} else {
				$this->attributes[ $key ][] = $value;
			}
		}
	}