MULTIPLE-MODES (DATA K &KEY START END KEY)
Returns the `k' most frequent elements of `data,' which should be a sequence. The algorithm involves sorting, and so the data must be numbers or the `key' function must produce numbers. Consider #'sxhash if no better function is available. Also returns the number of occurrences of each mode. The value is an association list of modes and their counts. This function is a little more computationally expensive than `mode,' so only use it if you really need multiple modes.