Goal:
  
  A : Type
  p : A → bool
  q : A → bool
  l : list A
  H0 : ∀ x : A, In x l → p x ↔ q x
  x : A
  H : In x l
  ============================
   (p x = q x)


DEBUG:Going to execute:
eapply eq_true_iff_eq