In math, a "combination" of a set of things is a subset of the things. We define the function combinations(things, k) to be a list of all the subsets of exactly k elements of things. Conceptually, that's all there is, but there are some questions to settle: (A) how do we represent a subset? (B) What order are the elements within each subset? (C) What order to we list the subsets? Here's what we will agree to: (A) a subset will be a list. (B) The order of elements within a list will be the same as the order within 'things'. So, for example, for combinations([1, 2, 3], 2) one of the subsets will be [1, 2]; whereas [2, 1] is not a subset. (C) The order of subsets will be lexicographical or sorted order -- that is, combinations([1, 2, 3], 2) returns [ [1, 2], [1, 3], 2, 3] ] because [1, 2] < [1, 3] < [2, 3]. You might want to use the function 'sorted' to make sure the results you return are properly ordered.