MrMage

public struct NonemptySequence<Element> {
    fileprivate let _storage: [Element]
    
    public init(first: Element, rest: [Element]) {
        _storage = [first] + rest
    }
}

extension NonemptySequence: Sequence {
    // call through to _storage here

MrMage

import Cocoa

struct TaskTitleArray: Equatable {
	let title: String
	let children: [TaskTitleArray]

	init(title: String, children: [TaskTitleArray]) {
		self.title = title
		self.children = children
	}