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       jQuery(function($) {

        // grab the initial top offset of the navigation
        var sticky_navigation_offset_top = $('#custom_nav_id').offset().top;

        // our function that decides weather the navigation bar should have "fixed" css position or not.
        var sticky_navigation = function(){
        var scroll_top = $(window).scrollTop(); // our current vertical position from the top

        // if we've scrolled more than the navigation, change its position to fixed to stick to top, otherwise change it back to relative
        if (scroll_top > sticky_navigation_offset_top) {
        $('#custom_nav_id').css({ 'position': 'fixed', 'top':0, 'width' : 'inherit', 'z-index' : '999'});
        } else {
        $('#custom_nav_id').css({ 'position': 'relative' });
        }
        };

        // run our function on load
        sticky_navigation();

        // and run it again every time you scroll
        jQuery(window).scroll(function($) {
        sticky_navigation();
        });

        });