docs: public static IntSet mutableFrom(Set<Integer> integerSet) {
        if (integerSet instanceof SmallIntSet) {
           return (SmallIntSet) integerSet;
        }
        if (integerSet instanceof ConcurrentSmallIntSet) {
           return (ConcurrentSmallIntSet) integerSet;
        }
        return mutableCopyFrom(integerSet);
     }