Goal:
  
  G : t
  s : VSet.t
  IH : forall m : nat,
       (m < VSet.cardinal s)%nat ->
       forall s : VSet.t,
       VSet.cardinal s = m ->
       forall (x y : V.t) (n : Z),
       lsp0 s x y = Some n -> exists p : SPath s x y, sweight p = n
  x : V.t
  y : V.t
  n : Z
  Hx : VSet.mem x s = true
  x' : Z
  n' : V.t
  H2 : In (x, x', n') (EdgeSet.elements (E G))
  p1 : SPath (VSet.remove x s) n' y
  H1 : x' + sweight p1 = n
  ============================
   (x' + sweight p1 = n)


DEBUG:Going to execute:
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"