join

In std/seq.

fn join<U>(self: Collection<T>, other: Collection<U>, by: (T, U) -> Bool) : List<Pair<T, U>>

Every pair of a value of self and a value of other that by relates, in the order of self, then of other: [1, 2] | join([1, 2, 3], (a, b) => a < b) gives the pairs (1, 2), (1, 3) and (2, 3).