//function to hide one element of the menu and adapt the button accordingly
function hide(el,img)
{
    el.style.display="none";
    if (img)
    {
        img.src='include/plus.png';
        img.alt='+';
    }
}

//function to show one element of the menu and adapt the button accordingly
function show(el,img)
{
    el.style.display="block";
    if (img)
    {
        img.src='include/minus.png';
        img.alt='-';
    }
}

//function to show or hide one element of the menu
function showhide (id)
{
    //get menu element and button for the element that has to be changed
    var el = document.getElementById(id);
    var img = document.getElementById(id+"button");
    //query current status and change it accordingly
    if (el.style.display != 'none')
        hide(el,img);
    else
        show(el,img);
}