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.In (x, e.π1, y) (E G1)
  ============================
   (EdgeSet.In (x, diff l x y, y) (WeightedGraph.E Gl))


DEBUG:Going to execute:
<coq-core.plugins.ltac::exact@0> $1
DEBUG LTAC Evaluated term: (EdgeSet.mem (x, e.π1, y) (E G1))
DEBUG LTAC Evaluated term: EdgeSet.mem_spec