Adds a new vertex to the graph if not present.

        :param vertex: Vertex
        :return: ``True`` if ``vertex`` added and not yet present. ``False`` otherwise.

def add_vertex(self, vertex):
        """Adds a new vertex to the graph if not present.

        :param vertex: Vertex
        :return: ``True`` if ``vertex`` added and not yet present. ``False`` otherwise.
        """
        if vertex not in self._vertices:
            self._vertices.add(vertex)
            return True

        return False