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
  pr1 : VSet.t
  pr0 : V.t
  pr2 : V.t
  a0 : SPath G pr1 pr0 pr2
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    let H1 := CoreTactics.block in
    Acc
      (fun
         x
          y : sigma
                (fun
                   index : sigma
                             (fun _ : VSet.t => sigma (fun _ : V.t => V.t))
                 =>
                 SPath G (Init.pr1 index) (Init.pr1 (Init.pr2 index))
                   (Init.pr2 (Init.pr2 index))) =>
       SPath_direct_subterm G (Init.pr1 (Init.pr1 x))
         (Init.pr1 (Init.pr2 (Init.pr1 x)))
         (Init.pr2 (Init.pr2 (Init.pr1 x))) (Init.pr1 (Init.pr1 y))
         (Init.pr1 (Init.pr2 (Init.pr1 y)))
         (Init.pr2 (Init.pr2 (Init.pr1 y))) (Init.pr2 x) 
         (Init.pr2 y))
      {|
        pr1 := {| pr1 := pr1; pr2 := {| pr1 := pr0; pr2 := pr2 |} |};
        pr2 := a0
      |})


DEBUG:Going to execute:
tac H
DEBUG LTAC Evaluated term: (let H := CoreTactics.block in
                            let H0 := CoreTactics.block in
                            let H1 := CoreTactics.block in
                            Acc
                              (fun
                                 x
                                  y : sigma
                                        (fun
                                           index : 
                                            sigma
                                              (fun _ : VSet.t =>
                                               sigma (fun _ : V.t => V.t)) =>
                                         SPath G (Init.pr1 index)
                                           (Init.pr1 (Init.pr2 index))
                                           (Init.pr2 (Init.pr2 index))) =>
                               SPath_direct_subterm G 
                                 (Init.pr1 (Init.pr1 x))
                                 (Init.pr1 (Init.pr2 (Init.pr1 x)))
                                 (Init.pr2 (Init.pr2 (Init.pr1 x)))
                                 (Init.pr1 (Init.pr1 y))
                                 (Init.pr1 (Init.pr2 (Init.pr1 y)))
                                 (Init.pr2 (Init.pr2 (Init.pr1 y)))
                                 (Init.pr2 x) (Init.pr2 y))
                              {|
                                pr1 :=
                                  {|
                                    pr1 := pr1;
                                    pr2 := {| pr1 := pr0; pr2 := pr2 |}
                                  |};
                                pr2 := a0
                              |})