Goal:
  
  G : t
  x : V.t
  y : V.t
  n : Z
  p : SPath (V G) x y
  Hp : sweight p = n
  l : labelling
  Hl : Z.of_nat (l x) + n <= Z.of_nat (l y)
  ============================
   (Z.of_nat (l x) + n <= Z.of_nat (l y))


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"