function ff_revoke_action(element, action)
{
    // check if user is logged in
    var userid = ff_getElementByName('userid').value;
    if (userid == 0) {
        alert('You are not logged in to the site.');
        return;
    } // if

    // search if userid is in the list
    var listid = ff_getIdByName('participants');
    var qrows = ff_queryRows[listid];
    var cnt = qrows.length;
    var i;
    var rowid = 0;
    for (i = 0; i < cnt; i++)
        if (qrows[i][1] == userid) {
            rowid = qrows[i][0];
            break;
        } // if
    if (rowid == 0) {
        alert('You have currently no registration.');
        return;
    } // if

    if (!confirm('Do you really want to revoke your registration?')) return;
    ff_getElementByName('deleteid').value = rowid;
    ff_submitForm();
} // ff_revoke_action