Flatten is a function that merges a set of provided `iter.Seq[T]`
values into a single `iter.Seq[T]` value. The values of each input are
yielded in the order yielded by each `iter.Seq[T]`, in the order provided
to the function.
Transform is a function that maps the values yielded by the input `seq`
to values produced by the input function `fn`, and returns an `iter.Seq`
that yields those new values.