module RGL::MutableGraph

to u and u will be adjacent to v.
will appear in the out-edges of v. Put another way, v will be adjacent
will appear in the out-edges of u and (u,v) (or equivalently (v,u))
after a call to the function #add_edge, this implies that edge (u,v)
Note that for undirected graphs, (u,v) is the same edge as (v,u), so

Inserts the edge (u,v) into the graph.

def add_edge(u, v)
  raise NotImplementedError
end