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
  ============================
   (VSet.For_all P s <-> VSet.for_all f s = true)


DEBUG:Going to execute:
etransitivity
DEBUG LTAC Evaluated term: iff_reflect