Unique combinations from a collection

I noticed there’s a method for counting the unique permutations of a collection - how come there is no equivalent for combinations? For example the unique combinations of length 2 of “aabc” are “aa”, “ab”, “ac”, “bc”. If others think this would be useful Id be interested in trying to implement it !

An equivalent would be `more_itertools.distinct_combinations(collection, k)` in Python

Locate the existing method for counting unique permutations in the Swift Algorithms repository and inspect its related tests. Compare the requested behavior with Python's distinct_combinations(collection, k), including the “aabc” length-2 example; done means the equivalent combinations operation is implemented and covered by tests.

Written by the indexing model from the issue text.