Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  y_0 : V.t
  x_0 : V.t
  Vx : VSet.In x_0 (V G)
  Vy : VSet.In y_0 (V G)
  Hxs : lsp G x_0 y_0 = None
  K : Z
  x : V.t
  k : Z
  p : SPath G (V G) x_0 x
  Hk : sweight p = k
  p' := sto_G' p : SPath G' (V G) x_0 x
  ============================
   ((y_0, K, ?y) = (y_0, K, x_0))


DEBUG:Going to execute:
let rec use_hyps :=
            match goal with
            | H:_ /\ _ |- _ => exact H || (destruct_hyp H; use_hyps)
            | H:(_ * _)%nat |- _ => exact H || (destruct_hyp H; use_hyps)
            | H:_ |- _ => solve [ inversion H ]
            | _ => idtac
            end
           in
           let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"