Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  e : EdgeSet.elt
  Hle : leq_vertices G n x e..t
  Vy : ~ VSet.In e..t (V G)
  nneg : n <= 0
  n0 : x <> e..t
  Hxs : lsp G x (s G) = None
  Vx : VSet.In x (V G)
  H : x <> s G
  K := Z.succ (- n - n) : Z
  G' := G' (s G) x K : t
  X : correct_labelling G (to_label ∘ lsp G' (s G'))
  HI' : invariants G'
  HG' : acyclic_no_loop G'
  H' : EdgeSet.In e (E G)
  n1 : e..s <> e..t
  ============================
   (EdgeSet.In e (E G))


DEBUG:Going to execute:
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"