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


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2