Function to compute k-Combinations of a list

kcombinations.groovy

List kCombinations(List l, Integer k) {
    assert k >= 0, 'Invalid "k"'
    if (l.empty || !k) []
    else if (k == 1) l.collate(1)
    else
        (0..l.size()-1).inject([] as Set) { acc, i ->
            def c = kCombinations(l.drop(i+1), k-1)
            acc + [l[i], c].combinations { cn ->
                def cnf = cn.flatten()
                cnf.size() == k ? cnf : [l[i],c].flatten()
            }
        }.toList()
}

assert kCombinations(1..4, 3).size() == 4
assert kCombinations(1..14, 4).size() == 1001