Returns a Boolean value that indicates whether this set is a strict subset of the given set.
- iOS 8.0+
- macOS 10.10+
- tvOS 9.0+
- watchOS 2.0+
- Xcode 8.0+
A set of the same type as the current set.
true if the set is a strict subset of
Set A is a strict subset of another set B if every member of A is also a member of B and B contains at least one element that is not a member of A.