Goal:
  
  A : Type
  P : A → Prop
  p : A → bool
  Hp : ∀ x : A, reflect (P x) (p x)
  x : A
  l : list A
  H : P x
  H0 : Forall P l
  IHForall : is_true (forallb p l)
  p0 : P x
  ============================
   (is_true true)


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