Goal:
  
  G : t
  HI : invariants
  P : VSet.elt -> Prop
  f : VSet.elt -> bool
  Hf : forall x : VSet.elt, ssrbool.reflect (P x) (f x)
  s : VSet.t
  x : VSet.elt
  Hx : VSet.In x s
  ============================
   (ssrbool.reflect (P x) (f x))


DEBUG:Going to execute:
(solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"