flat_map

In std/collections.

elementwise fn flat_map<T, U>(self: Collection<T>, by: T -> Seq<U>) : List<U>
elementwise fn flat_map<T, U>(self: Stream<T>, by: T -> Seq<U>) : Stream<U>

Each element through by to a sequence, and the members of those sequences in order: map(by) | emit. [1, 2] | flat_map(x => [x, x]) is [1, 1, 2, 2].