折りたたみ

var hideCache = new Object();

function $(id){
  return document.getElementById(id);
}

function hideSec(id){
  var elm = $(id);
  if(hideCache[id] == undefined || hideCache[id] == null){
    hideCache[id] = elm.innerHTML;
    elm.innerHTML = "";
  }
  else{
    elm.innerHTML = hideCache[id];
    hideCache[id] = null;
  }
}

<h1><a href="javascript:hideSec('section1');">1:Abst</a></h1>
<div id="section1">
  aaaaaaaaaaaa
</div>

<h1><a href="javascript:hideSec('section2');">2:Intro</a></h1>
<div id="section2">
  bbbbbbbbbbbbbbb
</div>

<h1><a href="javascript:hideSec('section3')">3:Concept</a></h1>
<div id="section3">
  cccccccccccccccccccccccc<br />
  ccccccccccccccccccccccccc
</div>