Goal:
  
  A : Type
  P : A → Type
  a : A
  l : list A
  x : A
  Hnth : Some a = Some x
  HPl : All P (a :: l)
  IHl : ∀ n : nat, nth_error l n = Some x → All P l → P x
  H0 : a = x
  ============================
   (P x)


DEBUG:Going to execute:
<coq-core.plugins.ltac::subst@1>