clause ::= (test-form form
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.