def get_neighbors_of(self, a):
return self.adjacency_lists.get(a, set())<EOL>
This will return the neighbors of the vertex

        Args:
            a (hash): is the vertex to get the neighbors for

        Returns:
            [] : a list of neighbors_of 'a'
                Will return an empty set if 'a' doesn't exist or has no
                neightbors.