Methods to iterate a Collection while exploiting Optional.

let test0 = [0, 1, 2, 3]
let first = test0.startingIndex
let second = first.flatMap { test0.elementIndex(after: $0) }
let third = second.flatMap { test0.elementIndex($0, offsetBy: +1) }
let fourth = test0.elementIndex(before: test0.endIndex)

first is not nil only if test0 isn't empty. second, third, and fourth are non-nil only if there is a subscriptable index at the desired offset. Instead of less-than comparisons with endIndex before indexing, you can use the existing Optional.none-testing infrastructure Swift already provides.