Goal:
    A : Type  P : A → Prop  Q : A → Prop  a : A  l' : list A
  ============================
   (All P []
    → All Q (a :: l')
      → 0 = S #|l'| → All2 (λ x y : A, P x ∧ Q y) [] (a :: l'))


DEBUG:Going to execute:
hnf