Adds the specified vertex to the set of adjacencies. Note that this method
only adds a directed adjacency. If the vertex belongs to an
undirected graph, the graph will be put into an inconsistent state if an
adjacency (u, v) is added using this method without also adding the directed
adjacency (v, u)!
Removes all vertices from the adjacency set. The adjacency set will be
empty after this call returns. The cautionary statements regarding the use
of removeAdjacency() in an undirected graph also apply to this method.