Constructs a new, empty list set, sorted according to the specified
comparator. All elements inserted into the set must be mutually
comparable by the specified comparator: comparator.compare(e1,
e2) must not throw a ClassCastException for any elements
e1 and e2 in the set. If the user attempts to add
an element to the set that violates this constraint, the
add call will throw a ClassCastException.
comparator - the comparator that will be used to order this set.
If null, the natural
ordering of the elements will be used.