reduce

In std/collections.

fn reduce<T, R>(self: Collection<T>, initial: R, by: (R, T) -> R) : R
fn reduce<T>(self: Collection<T>, by: (T, T) -> T) : Option<T>

The elements folded from the left by by, from initial, or without it from the first element, the result then none over []: [1, 2, 3] | reduce(0, (a, x) => a + x) is 6. The checker rejects a Set, which has no positions to fold in.