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:
case H; clear H; do_intro
Level 4: evaluation returns
case H; clear H; do_intro; do_intro
where
use_hyp := <tactic closure>
of type
tacvalue
do_intro := <tactic closure>
of type
tacvalue
destruct_hyp := <tactic closure>
of type
tacvalue
H := ine
of type
constr of type tacvalue