Goal:
  
  G : t
  invG : invariants G
  acG : acyclic_no_loop G
  x : V.t
  y : V.t
  hx : VSet.In x (V G)
  hy : VSet.In y (V G)
  nxy : Z
  hxy : (lsp G x y <= Some nxy)%nbar
  k : nat
  _p_ : positive
  H : 0 <= Z.neg _p_
  ============================
   (k =
    match
      match Z.of_nat k with
      | 0 => Z.neg _p_
      | Z.pos y' => Z.pos_sub y' _p_
      | Z.neg y' => Z.neg (_p_ + y')
      end
    with
    | Z.pos p => Pos.to_nat p
    | _ => 0%nat
    end)


DEBUG:Going to execute:
<coq-core.plugins.ltac::contradiction@0> $1
Level 0: Backtrace:

No primitive equality found.