Goal:
  
  A : Type
  P : A → Prop
  Q : A → Prop
  p : A → bool
  x : A
  l : list A
  H : Q x
  HQ : Forall Q l
  Hp : ∀ x : A, Q x → reflect (P x) (p x)
  IHHQ : forallb p l = true → Forall P l
  pa : is_true true
  pl : is_true (forallb p l)
  p0 : P x
  ============================
   (P x)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]