Returns a Boolean value that indicates whether the set is a subset of another set.
- iOS 7.0+
- macOS 10.9+
- 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 subset of
Set A is a subset of another set B if every member of A is also a member of B.