Closure for returning the combinations of the input list of the provided size, r

Combinations.groovy

// returns combinations of the input list of the provided size, r
def combinationsOf = { List list, int r ->
    assert (0..<list.size()).contains(r)
    def combs = [] as Set
    list.eachPermutation {
        combs << it.subList(0, r).sort { a, b -> a <=> b }
    }
    combs as List
}

assert combinationsOf([1, 2, 3], 2) == [[1, 2], [1, 3], [2, 3]]
assert combinationsOf([1, 2, 3, 4], 2) == [[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]
assert combinationsOf([1, 2, 3, 4], 3) == [[1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4]]