Goal:
  
  G : t
  HI : invariants
  z : V.t
  hin : VSet.In z (V G)
  x : V.t
  y : V.t
  x0 : Z
  ine : VSet.In (x, x0, y)..s (V G) /\ VSet.In (x, x0, y)..t (V G)
  p : PathOf y z
  IHp : VSet.In z (V G) -> VSet.In y (V G)
  ============================
   (VSet.In x (V 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"