Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  HPl : is_true (forallb p (a :: l))
  ============================
   ((∀ n : nat, nth_error l n = Some a → forallb p l → p a)
    → let H := CoreTactics.block in p a)


DEBUG:Going to execute:
<coq-core.plugins.ltac::intro@0>