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
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]