public function is_singleton( $id ) {
		if ( ! is_string( $id ) ) {
			throw new InvalidArgumentException( 'Invalid argument. Must be string.' );
		}

		if ( ! $this->bound( $id ) ) {
			return false;
		}

		$id = $this->get_class_prefix( $id );

		return $this->bindings[$id]['singleton'] === true;
	}