   /// - Returns: `true` if the set is a subset of `possibleSuperset`;
   ///   otherwise, `false`.
   @inlinable
-  public func isSubset<S: Sequence>(of possibleSuperset: __owned S) -> Bool
+  public func isSubset<S: Sequence>(of possibleSuperset: S) -> Bool
   where S.Element == Element {
     // FIXME(performance): isEmpty fast path, here and elsewhere.
     let other = Set(possibleSuperset)

  /// - Returns: `true` if the set is a strict superset of
  ///   `possibleStrictSubset`; otherwise, `false`.
  @inlinable
  public func isStrictSuperset<S: Sequence>(
    of possibleStrictSubset: __owned S
  ) -> Bool
  where S.Element == Element {
    let other = Set(possibleStrictSubset)
    return other.isStrictSubset(of: self)

  /// - Returns: `true` is the set is strict subset of
  ///   `possibleStrictSuperset`; otherwise, `false`.
  @inlinable
  public func isStrictSubset<S: Sequence>(of possibleStrictSuperset: S) -> Bool
  where S.Element == Element {
    // FIXME: code duplication.
    let other = Set(possibleStrictSuperset)
