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


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