function getRefToDiv(divID,oDoc) {
  if( document.getElementById ) {
    return document.getElementById(divID); }
  if( document.all ) {
    return document.all[divID]; }
  if( !oDoc ) { oDoc = document; }
  if( document.layers ) {
    if( oDoc.layers[divID] ) { return oDoc.layers[divID]; } else {
      //repeatedly run through all child layers
      for( var x = 0, y; !y && x < oDoc.layers.length; x++ ) {
        //on success, return that layer, else return nothing
        y = getRefToDiv(divID,oDoc.layers[x].document); }
    return y; } }
  return false;
}

function changeDisplay( elementId, setTo ) {
   var theElement;
	 theElement = getRefToDiv(elementId);
  if( !theElement ) {
    /* The page has not loaded, or the browser claims to
    support document.getElementById or document.all but
    cannot actually use either */
    return;
  }
  //Reference the style ...
  if( theElement.style ) { theElement = theElement.style; }
  if( typeof( theElement.display ) == 'undefined' ) {
    //The browser does not allow us to change the display style
    //Alert something sensible (not what I have here ...)
    //window.alert( 'Your browser does not support this' );
    return;
  }
  //Change the display style
  theElement.display = setTo;
}

function hideElement( elementId ) {
  changeDisplay( elementId, 'none' );
	return;
}

function showElement( elementId ) {
  changeDisplay( elementId, 'block' );
	return;
}

function contextMenu( elementId ) {
  hideElement('welcome');
	hideElement('choose');
	hideElement('book');
	hideElement('guide');
	showElement(elementId);
	return;
}