func Combinations(iterable []string, r int) (rt [][]string) {
	pool := iterable
	n := len(pool)

	if r > n {
		return
	}

	indices := make([]int, r)
	for i := range indices {
		indices[i] = i
	}

	result := make([]string, r)
	for i, el := range indices {
		result[i] = pool[el]
	}
	s2 := make([]string, r)
	copy(s2, result)
	rt = append(rt, s2)

	for {
		i := r - 1
		for ; i >= 0 && indices[i] == i+n-r; i -= 1 {
		}

		if i < 0 {
			return
		}

		indices[i] += 1
		for j := i + 1; j < r; j += 1 {
			indices[j] = indices[j-1] + 1
		}

		for ; i < len(indices); i += 1 {
			result[i] = pool[indices[i]]
		}
		s2 = make([]string, r)
		copy(s2, result)
		rt = append(rt, s2)
	}

}

func Combinations2(set []string, n int) (subsets [][]string) {
	length := uint(len(set))

	if n > len(set) {
		n = len(set)
	}

	// Go through all possible combinations of objects
	// from 1 (only first object in subset) to 2^length (all objects in subset)
	for subsetBits := 1; subsetBits < (1 << length); subsetBits++ {
		if n > 0 && bits.OnesCount(uint(subsetBits)) != n {
			continue
		}

		var subset []string

		for object := uint(0); object < length; object++ {
			// checks if object is contained in subset
			// by checking if bit 'object' is set in subsetBits
			if (subsetBits>>object)&1 == 1 {
				// add object to subset
				subset = append(subset, set[object])
			}
		}
		// add subset to subsets
		subsets = append(subsets, subset)
	}
	return subsets
}

func All(set []string) (subsets [][]string) {
	length := uint(len(set))

	// Go through all possible combinations of objects
	// from 1 (only first object in subset) to 2^length (all objects in subset)
	for subsetBits := 1; subsetBits < (1 << length); subsetBits++ {
		var subset []string

		for object := uint(0); object < length; object++ {
			// checks if object is contained in subset
			// by checking if bit 'object' is set in subsetBits
			if (subsetBits>>object)&1 == 1 {
				// add object to subset
				subset = append(subset, set[object])
			}
		}
		// add subset to subsets
		subsets = append(subsets, subset)
	}
	return subsets
}