Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  n : Z
  x : V.t
  y : VSet.elt
  Vy : VSet.In y (V G)
  Hle : leq_vertices G n x y
  Vx : ~ VSet.In x (V G)
  n0 : s G <> x
  ============================
   (invariants 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"