Goal:
  
  A : Type
  p : A → bool
  l : list A
  n : nat
  x : A
  Hnth : nth_error l n = Some x
  HPl : is_true (forallb p l)
  ============================
   (is_true (p x))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrtclintros@0> arg