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                <div class="boxes">
  <input id="box0" type="checkbox" />
  <input id="box1" type="checkbox" />
  <input id="box2" type="checkbox" />
  <input id="box3" type="checkbox" />
  <input id="box4" type="checkbox" />
  <input id="box5" type="checkbox" />
  <input id="box6" type="checkbox" />
  <input id="box7" type="checkbox" />
  <input id="box8" type="checkbox" />
  <input id="box9" type="checkbox" />


                input[type=checkbox] {
   display: block;
   margin: 10px;


                //Set up out lastBox variable, lastBox is one of the extermities.
var lastBox = -1;

//Selects a range of checkboxes if it can. elm is a event object.
function selectRange(elm) {
  //Set up some variables so we can select some checkboxes!
  var newId = parseInt("box", ""));
  var checkboxes = document.querySelectorAll(".boxes input[type=checkbox]");
  //Swap lastBox and newId if lastBox is larger
  if (lastBox > newId) {
    var hold = newId;
    newId = lastBox;
    lastBox = hold;
  //Check if there's a starting pointer and that shift is being held
  if (lastBox != -1 && elm.shiftKey == true) {
    //Start at the lastBox element and check every box until the newId element.
    for (var i=lastBox; i<=newId; i++)
      checkboxes[i].checked = true;
    lastBox = -1;
  } else
    //If it's not, set lastBox to the current element id.
    lastBox = parseInt("box", ""));

//Get all the checkboxes
var boxes = document.querySelectorAll(".boxes input[type=checkbox]");
//Loop through them all and give them an event listener [NB: There's probably a better way to do this!]
for (var i = 0; i<boxes.length; i++)
  boxes[i].addEventListener('click', selectRange, true);