multiple-modes (data k &key start end key)
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'<br> function must produce numbers. Consider #'sxhash if no better function is<br> available. Also returns the number of occurrences of each mode. The value is<br> an association list of modes and their counts. This function is a little more<br> computationally expensive than mode,' so only use it if you really need