r1.swift

extension Sequence {
  typealias E = Iterator.Element
  typealias I = Iterator

  public func prefix(while predicate: (E) -> Bool) -> UnfoldSequence<E, I> {
    return sequence(state: makeIterator(), next: { (s: inout I) -> E? in
      guard let next = s.next() else {
        return nil
      }
      return predicate(next) ? next : nil
    })
  }
}