Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  y : V.t
  Vy : ~ VSet.In y (V G)
  Hle : forall l : labelling,
        correct_labelling G l -> Z.of_nat (l x) + n <= Z.of_nat (l y)
  nneg : n <= 0
  n0 : x <> y
  z : Z
  H : lsp G x (s G) = Some z
  mem : VSet.In x (V G)
  G' := Subgraph1.G' G (s G) x z : t
  X : correct_labelling G (to_label ∘ lsp G' (s G'))
  HI' : invariants G'
  HG' : acyclic_no_loop G'
  e : EdgeSet.elt
  H' : EdgeSet.In e (E G)
  n1 : e..s <> y
  n2 : e..t <> y
  ============================
   (EdgeSet.In e (E G))


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"