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 G1 x y
  __view_subject__ : EdgeSet.In (x, e.π1, y) (E G1)
  ============================
   (((x, e.π1, y)..s, diff l (x, e.π1, y)..s y, y) = (x, diff l x y, y))


DEBUG:Going to execute:
trivial; hnf