cond {↓clause}* → {result}*

test-form — a form.

forms — an implicit progn.

results — the values of the forms in the first clause whose test-form yields true, or the primary value of the test-form if there are no forms in that clause, or else nil if no test-form yields true.

cond allows the execution of forms to be dependent on test-form.

Test-forms are evaluated one at a time in the order in which they are given in the argument list until a test-form is found that evaluates to true.

If there are no forms in that clause, the primary value of the test-form is returned by the cond form. Otherwise, the forms associated with this test-form are evaluated in order, left to right, as an implicit progn, and the values returned by the last form are returned by the cond form.

Once one test-form has yielded true, no additional test-forms are evaluated. If no test-form yields true, nil is returned.

(defun select-options () 
  (cond ((= a 1) (setq a 2)) 
        ((= a 2) (setq a 3)) 
        ((and (= a 3) (floor a 2))) 
        (t (floor a 3)))) → SELECT-OPTIONS 
(setq a 1) → 1 
(select-options) → 2 
a → 2 
(select-options) → 3 
a → 3 
(select-options) → 1 
(setq a 5) → 5 
(select-options) → 1, 2

if, case.