cond allows the execution of forms to be dependent
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.