pub trait VariableVertexes<T, E, K, V, W, C>: DirectedGraph<T, E, K, V, W, C> where
K: Hash + Eq + Clone,
C: Hash + Eq + Clone,
W: Sum + Eq + Ord + Copy,
T: Vertex<K, V>,
E: Edge<K, W, C>, {
fn add_vertex(&mut self, vertex: T);
fn remove_vertex(&mut self, key: K);
}