function relative(aRoot, aPath) {
	  if (aRoot === "") {
	    aRoot = ".";
	  }

	  aRoot = aRoot.replace(/\/$/, '');

	  // It is possible for the path to be above the root. In this case, simply
	  // checking whether the root is a prefix of the path won't work. Instead, we
	  // need to remove components from the root one by one, until either we find
	  // a prefix that fits, or we run out of components to remove.
	  var level = 0;
	  while (aPath.indexOf(aRoot + '/') !== 0) {
	    var index = aRoot.lastIndexOf("/");
	    if (index < 0) {
	      return aPath;
	    }

	    // If the only part of the root that is left is the scheme (i.e. http://,
	    // file:///, etc.), one or more slashes (/), or simply nothing at all, we
	    // have exhausted all components, so the path is not relative to the root.
	    aRoot = aRoot.slice(0, index);
	    if (aRoot.match(/^([^\/]+:\/)?\/*$/)) {
	      return aPath;
	    }

	    ++level;
	  }

	  // Make sure we add a "../" for each component we removed from the root.
	  return Array(level + 1).join("../") + aPath.substr(aRoot.length + 1);
	}
Make a path relative to a URL or another path.

@param aRoot The root path or URL.
@param aPath The path or URL to be made relative to aRoot.