// BlockCollapse.js
  
  // Add a button before the specified element (assumed
  // to be block style) that will make the element
  // disappear when clicked once and re-appear when
  // clicked a second time.
  // The button is placed within a div to ensure that
  // the markup we generate is valid XHTML.
  
  function makeCollapsible(elementId) {
    var element = window.document.getElementById(elementId);
    if (element) {
      var div = window.document.createElement("div");
      element.parentNode.insertBefore(div, element);
      var button = window.document.createElement("button");
      div.appendChild(button);
      button.setAttribute("type", "button");
      var buttonText = window.document.createTextNode("Click to collapse");
      button.appendChild(buttonText);
      button.setAttribute("onclick", 
                          "toggleVisibility(this,'" + elementId + "');");
    }
    return;
  }
  
  // Function called when the button is clicked.
  
  function toggleVisibility(button, elementId) { 
    var element = window.document.getElementById(elementId);
    if (element) {
      if (element.style.display == "none") {
        element.style.display = "block";
        button.childNodes[0].data = "Click to collapse";
      } else {
        element.style.display = "none";
        button.childNodes[0].data = "Click to expand";
      }
    }
    return;
  }