difference method
override
Returns a new set with the the elements of this that are not in other.
That is, the returned set contains all the elements of this Set that are
not elements of other according to other.contains.
Note that the returned set will use the default equality operation, which may be different than the equality operation this uses.
Implementation
Set<V> difference(Set<Object> other) =>
where((element) => !other.contains(element)).toSet();