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)
  HGl : correct_labelling G1 l
  H0 : invariants G2
  H1 : acyclic_no_loop G2
  embed : forall v1 v2 : VSet.elt,
          VSet.In v1 (V G1) ->
          VSet.In v2 (V G1) -> (lsp G2 v1 v2 <= lsp G1 v1 v2)%nbar
  G : t
  s0 : VSet.t
  x : V.t
  y : sigma
        (fun index : sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t)) =>
         SPath G (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  H2 : SPath_direct_subterm G (pr1 (pr1 y)) (pr1 (pr2 (pr1 y)))
         (pr2 (pr2 (pr1 y)))
         (pr1
            (pr1
               {|
                 pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
                 pr2 := spath_refl G s0 x
               |}))
         (pr1
            (pr2
               (pr1
                  {|
                    pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
                    pr2 := spath_refl G s0 x
                  |})))
         (pr2
            (pr2
               (pr1
                  {|
                    pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
                    pr2 := spath_refl G s0 x
                  |}))) (pr2 y)
         (pr2
            {|
              pr1 := {| pr1 := s0; pr2 := {| pr1 := x; pr2 := x |} |};
              pr2 := spath_refl G s0 x
            |})
  ============================
   (Acc
      (fun
         x
          y : sigma
                (fun
                   index : sigma
                             (fun _ : VSet.t => sigma (fun _ : V.t => V.t))
                 => SPath G (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
       =>
       SPath_direct_subterm G (pr1 (pr1 x)) (pr1 (pr2 (pr1 x)))
         (pr2 (pr2 (pr1 x))) (pr1 (pr1 y)) (pr1 (pr2 (pr1 y)))
         (pr2 (pr2 (pr1 y))) (pr2 x) (pr2 y)) y)


DEBUG:Going to execute:
match type of H with
           | sigma (fun x => _) =>
               let H' := fresh H in
               destruct H as [? H']; destruct_right_sigma H'
           | sigma _ =>
               let H' := fresh H in
               destruct H as [? H']; destruct_right_sigma H'
           | _ => idtac
           end