code: public static IntSet mutableFrom(Set<Integer> integerSet) {
        if (integerSet instanceof SmallIntSet) {
           return (SmallIntSet) integerSet;
        }
        if (integerSet instanceof ConcurrentSmallIntSet) {
           return (ConcurrentSmallIntSet) integerSet;
        }
        return mutableCopyFrom(integerSet);
     }
docs: Returns an IntSet that is mutable that contains all of the values from the given set. If this provided Set is
  already an IntSet and mutable it will return the same object.
  @param integerSet set to use values from
  @return IntSet that is mutable with the values set