clause::= (test-form {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.

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