Goal:
  
  G1 : t
  G2 : t
  l : labelling
  Gl := relabel_on G1 G2 l : t
  H : invariants G1
  preserves_edges : forall e : EdgeSet.elt,
                    EdgeSet.In e (E G1) -> EdgeSet.In e (E G2)
  x : V.t
  y : V.t
  e : EdgeOf G2 x y
  E : EdgeSet.mem (x, e.π1, y) (E G1) = false
  ============================
   (EdgeSet.In (relabel_map G1 l (x, e.π1, y))
      (edge_map (relabel_map G1 l) (WeightedGraph.E G2)))


DEBUG:Going to execute:
apply : edge_map_spec1