Indexへの制約を外したのでStringに直接使える #CodePiece

CodePiece.swift

extension Collection {
    
    func list3() -> [[Element]] {
        
        zip(0..., self).flatMap { (i: Int, e0: Element) -> [[Element]] in
            zip((i + 1)..., self[index(startIndex, offsetBy: (i + 1))...]).flatMap { (j: Int, e1: Element) -> [[Element]] in
                self[index(startIndex, offsetBy: (j + 1))...].map { (e2: Element) -> [Element] in
                    [e0, e1, e2]
                }
            }
        }
    }
}