NonemptySequence.swift

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
}

extension NonemptySequence where Element: Equatable {
    public static func ==(A: NonemptySequence, B: NonemptySequence) -> Bool {
        return A._storage == B._storage
    }
}