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
  ============================
   (EdgeSet.In
      (x, if EdgeSet.mem (x, e.π1, y) (E G1) then diff l x y else e.π1, y)
      (E Gl))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrcase@0> $1 $2
DEBUG LTAC Evaluated term: (if EdgeSet.mem (x, e.π1, y) (E G1)
                            then diff l x y
                            else e.π1)