Goal:
  
  G1 : t
  G2 : t
  l : labelling
  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
  a : sigma
        (fun index : sigma (fun _ : VSet.t => sigma (fun _ : V.t => V.t)) =>
         SPath G (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  ============================
   (Acc (SPath_subterm G) a)


DEBUG:Going to execute:
Tactics.autoinjection ltac:(Tactics.inject)
Level 4: Backtrace:

Tactic failure (level 2).
Level 4: Backtrace:

Tactic failure (level 1).
Level 3: Backtrace:

Tactic failure (level 1).
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

Tactic failure.
Level 0: Backtrace:

Tactic failure.