Goal:
  
  A : Type
  P : A → Prop
  x : A
  l : list A
  H : P x
  HPl : Forall P l
  IHHPl : ∀ n : nat, nth_error l n = Some x → P x
  ============================
   (P x)


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