window.confirm em a4j:commandButton

<a4j:commandButton
        reRender="rdtFollowp"
        onclick="if (! window.confirm('Confirma?') ) {return false}"
        actionListener="#{processoImportacaoMB.deleteRegFlp}"
        image="/imagens/ImagensUtilizadas/16x16/remove.png"
        title="Excluir" >  
    <a4j:actionparam id="deleteRegFlp" value="#{flp.id}" name="pDelteteFlp"/>
</a4j:commandButton>