/**
 * This debug function displays plain-text debugging messages in a
 * special box at the end of a document.  It is a useful alternative
 * to using alert() to display debugging messages.
 **/
function debug(msg) {
    // If we haven't already created a box within which to display
    // our debugging messages, then do so now.  Note that to avoid
    // using another global variable, we store the box node as a property
    // of this function.
    if (!debug.box) {
        // Create a new <div> element
        debug.box = document.createElement("div");
        // Specify what it looks like using CSS style attributes
        debug.box.setAttribute("style", 
                               "background-color: white; " +
                               "font-family: monospace; " +
                               "border: solid black 3px; " +
                               "padding: 10px;");
        
        // And append our new <div> element to the end of the document
        document.body.appendChild(debug.box);

        // Now add a title to our <div>.  Note that the innerHTML property is
        // used to parse a fragment of HTML and insert it into the document.
        // innerHTML is not part of the W3C DOM standard, but it is supported
        // by Netscape 6 and Internet Explorer 4 and later.  We can avoid 
        // the use of innerHTML by explicitly creating the <h1> element,
        // setting its style attribute, adding a Text node to it, and 
        // inserting it into the document, but this is a nice shortcut
        debug.box.innerHTML =
            "<h1 style='text-align:center'>Debugging Output</h1>";
    }

    // When we get here, debug.box refers to a <div> element into which
    // we can insert our debugging message.
    // First, create a <p> node to hold the message
    var p = document.createElement("p");
    // Now create a text node containing the message, and add it to the <p>
    p.appendChild(document.createTextNode(msg));
    // And append the <p> node to the <div> that holds the debugging output
    debug.box.appendChild(p);
}