pub fn map2(
  opt_a: Option<a>,
  opt_b: Option<b>,
  f: fn(a, b) -> result,
) -> Option<result> {
  when opt_a is {
    None ->
      None
    Some(a) ->
      when opt_b is {
        None ->
          None
        Some(b) ->
          Some(f(a, b))
      }
  }
}

test map2_3() {
  map2(Some(14), Some(42), fn(a, b) { (a, b) }) == Some((14, 42))
}