window.onload=rollup;

function rollup()
{
    // Check we're working with a DOM compliant browser
    if (document.getElementById && document.createElement)
    {
        var objMenu = document.getElementById('mainnav');

        var objNested = objMenu.getElementsByTagName('ul');

        // Hide each of the nested unordered list
        for (var i=0; i<objNested.length; i++)
            objNested[i].style.display = 'none';
    }
}