function toggleCommentForm (pfx) {
    var oBlock = document.getElementById (pfx + '_invisible');
    var oLink  = document.getElementById (pfx + '_visible');
    if (oBlock) {
        if (oBlock.style.display == 'none') {
            oLink.style.display = 'none';
            oBlock.style.display = 'block';
        } else {
            oLink.style.display = 'block';
            oBlock.style.display = 'none';
        }
    }
}