code: public static <E> TreeSet<E> newTreeSet(Comparator<? super E> comparator) {
      return new TreeSet<E>(checkNotNull(comparator));
    }
docs: Creates a <i>mutable</i>, empty {@code TreeSet} instance with the given comparator.
  
  <p><b>Note:</b> if mutability is not required, use {@code
  ImmutableSortedSet.orderedBy(comparator).build()} instead.
  
  <p><b>Note for Java 7 and later:</b> this method is now unnecessary and should be treated as
  deprecated. Instead, use the {@code TreeSet} constructor directly, taking advantage of the new
  <a href="http://goo.gl/iz2Wi">"diamond" syntax</a>. One caveat to this is that the {@code
  TreeSet} constructor uses a null {@code Comparator} to mean "natural ordering," whereas this
  factory rejects null. Clean your code accordingly.
  
  @param comparator the comparator to use to sort the set
  @return a new, empty {@code TreeSet}
  @throws NullPointerException if {@code comparator} is null