function raw_popup(url, target, features) {
    // pops up a window containing url optionally named target, optionally having features
    if (isUndefined(features)) features = _POPUP_FEATURES;
    if (isUndefined(target  )) target   = '_blank';
    var theWindow = window.open(url, target, features);
    theWindow.focus();
    return theWindow;
}

function link_popup(src, features) {
    // to be used in an html event handler as in: <a href="..." onclick="link_popup(this,...)" ...
    // pops up a window grabbing the url from the event source's href
    return raw_popup(src.getAttribute('href'), src.getAttribute('target') || '_blank', features);
}

function isUndefined(v) {
    var undef;
    return v===undef;
}