type Iterable[T] = concept x for v in x: type(v) is T proc map2[T, U](itit: Iterable[Iterable[T]], f: proc(_: T): U): seq[U] = result = @[] for it in itit: for v in it: result.add(f(v)) echo map2(@[@[1, 2, 3], @[4, 5, 6]], proc (x: int): int = x + 1)