Goal:
  
  A : Type
  p : A → bool
  n : nat
  x : A
  Hnth : nth_error [] n = Some x
  HPl : is_true (forallb p [])
  ============================
   (p x = true)


DEBUG:Going to execute:
match goal with
           | H:¬ _ |- _ => solve [ case H; trivial ]
           end
Level 1: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.
Level 0: Backtrace:

No such assumption.