     public func rotated(_ n: Int) -> ConcatBidirectionalCollection<SubSequence, SubSequence> {
         let count: Int = numericCast(self.count)
         if count == 0 {
-            return self.subSequence.concat(self.subSequence)
+            return self[...].concat(self[...])
         }
         if n < 0 {
             let _n = -n % count

    public func rotated(_ n: Int) -> ConcatCollection<SubSequence, SubSequence> {
        let count: Int = numericCast(self.count)
        if count == 0 {
            return self.subSequence.concat(self.subSequence)
        }
        if n < 0 {
            let _n = -n % count

//  THE SOFTWARE.
//

extension Sequence where Element : Equatable {
    
    /// Return `true` if all of elements in `seq` is `x`.
