group-by
builtin-function
(group-by key seq)
Collects the elements of seq that have the same key into subsequences, as
determined by the function key. Keys should be numbers, strings or symbols.
make-html > (group-by length '("aap" "noot" "mies" "blablabla"))
it: ((3 ("aap")) (4 ("noot" "mies")) (9 ("blablabla")))