checked attribute for a checkbox input

Hello

Can somebody help me on this point :

How to build with AllegroServe a form with an input type "checkbox" where
the boolean attribute checked is set depending on the evaluation of a lisp
function.

This does'nt seem to work :
((:input :type "checkbox" :name "foo" :value "yes" :checked
(a-lisp-function)  ))

Regards

Jacques RATTIER

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