def add(self, key):
        """Add a new vertex to the graph.
        """
        if key in self._vertices:
            raise ValueError("vertex exists")
        self._vertices.add(key)
        self._forwards[key] = set()
        self._backwards[key] = set()
Add a new vertex to the graph.
        
product/gradle-plugin/src/main/python/pip/_vendor/resolvelib/structs.py