docs: public static IntSet from(Set<Integer> integerSet) {
        if (integerSet instanceof IntSet) {
           return (IntSet) integerSet;
        }
        int size = integerSet.size();
        switch (size) {
           case 0:
              return EmptyIntSet.getInstance();
           case 1:
              return new SingletonIntSet(integerSet.iterator().next());
           default:
              return new SmallIntSet(integerSet);
        }
     }