function checkvisible(){
    var prev = document.getElementById("ln_prev");
    var next = document.getElementById("ln_next");
    if(prev && next){
        prev.style.display = (MDS_CURRENT > 0)?'':'none';
        next.style.display = (MDS_CURRENT < MDS_CONTENTS.length-1)?'':'none';
    }
}

function nextcontent(){
    var curr = document.getElementById(MDS_CONTENTS[MDS_CURRENT]);
    var next = document.getElementById(MDS_CONTENTS[++MDS_CURRENT]);
    curr.className = '';
    curr.setAttribute('class',curr.className);
    next.className = 'selected'
    next.setAttribute('class',next.className);
    checkvisible();
}

function prevcontent(){
    var curr = document.getElementById(MDS_CONTENTS[MDS_CURRENT]);
    var next = document.getElementById(MDS_CONTENTS[--MDS_CURRENT]);
    curr.className = '';
    curr.setAttribute('class',curr.className);
    next.className = 'selected'
    next.setAttribute('class',next.className);
    checkvisible();
}