If ensure's predicate evaluates to false, then it will generate a
test failure. You can use the report and arguments keyword parameters
to customize the report generated in test results. For example:

(ensure (= 23 12)
:report “I hope ~a does not = ~a”
:arguments (12 23))

will generate a message like

Warning: Ensure failed: (= 23 12) (I hope 12 does not = 23)