Goal:
  
  A : Type
  B : Type
  x : A
  xs : list A
  ih : ∀ l' : list B, map2 (λ (x : A) (_ : B), x) xs l' = firstn #|l'| xs
  y : B
  ys : list B
  ============================
   (x :: map2 (λ (x : A) (_ : B), x) xs ys = x :: firstn #|ys| xs)


DEBUG:Going to execute:
trivial; hnf; intros