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    $.fn.isOnScreen = function(){
        var win = $(window);
        var viewport = { top : win.scrollTop() };
        viewport.bottom = viewport.top + win.height() - 120;
        var bounds = this.offset();
        if(bounds){ 
            bounds.bottom = bounds.top + this.outerHeight() + 120;
            return (!(viewport.bottom < bounds.top || viewport.top > bounds.bottom));
        }
    }

    $.fn.changeGo = function(){
        if($(this).length > 0){
            if($(this).isOnScreen() === true){
                $(this).addClass('go').removeClass('nogo');
            }else{
                $(this).removeClass('go').addClass('nogo');
            }
        }
    };

    function go(){
        $('section').each(function(){ $(this).changeGo(); });
    }

    $('section').addClass('nogo');

    $(document).on('scroll ready', function(){
        go();
    });