code: public static IntSet mutableCopyFrom(Set<Integer> mutableSet) {
        if (mutableSet instanceof SingletonIntSet) {
           return mutableSet(((SingletonIntSet) mutableSet).value);
        }
        return new SmallIntSet(mutableSet);
     }
docs: Returns an IntSet that contains all ints from the given Set that is mutable. Updates to the original Set or
  the returned IntSet are not reflected in the other.
  @param mutableSet set to copy from
  @return IntSet with the values set