Goal:
  
  A : Type
  p : A → bool
  a : A
  l : list A
  IHl : ∀ n : nat, nth_error l n = Some a → forallb p l → p a
  ============================
   (p a ∧ forallb p l → p a)


DEBUG:Going to execute:
symmetry; trivial
Level 1: Backtrace:

In environment
A : Type
p : A → bool
a : A
l : list A
IHl : ∀ n : nat, nth_error l n = Some a → forallb p l → p a
_view_subject_ : p a ∧ forallb p l
Unable to unify "true" with "p a".
Level 0: Backtrace:

In environment
A : Type
p : A → bool
a : A
l : list A
IHl : ∀ n : nat, nth_error l n = Some a → forallb p l → p a
_view_subject_ : p a ∧ forallb p l
Unable to unify "true" with "p a".