mode (data &key start end key)
Returns the most frequent element of data,' which should be a sequence. The<br> 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 the mode. If there is
more than one mode, this returns the first mode, as determined by the sorting of
the numbers.