Lisp insertion sort -> defun span() function confusion

Question

I am trying to learn how lisp works. Below is a part of a insertion sort algorithm and I don't understand what it is doing as a whole. I kind of understand what predicate list is but not sure what it is checking. Could someone explain?

(defun span (predicate list)
  (let ((tail (member-if-not predicate list)))
    (values (ldiff list tail) tail)))

Answer 1

(member-if-not p l) returns a tail of l beginning with the first element for which p is false. So (member-if-not #'evenp '(2 3 4)) is (3 4) (and this is eq to the cdr of the original list in this case).

If l2 is a tail of l1 then (ldiff l1 l2) returns a list of the elements of l1 which precede it.

So

(let ((l '(2 3 4)))
  (let ((tail (member-if-not #'evenp l)))
    (values (ldiff l tail) tail)))

will return (2) and (3 4) (and the second value will be the cdr of the original list).