function dir(elem, dir, until) {    //这个dir怎么理解？until呢？
  var matched = [],
    truncate = until !== undefined;
  while ((elem = elem[dir]) && elem.nodeType !== 9) { 
    if (elem.nodeType === 1) {
      if (truncate) {
        if (elem.nodeName.toLowerCase() == until || elem.className == until) {
          break;
        }
      }
      matched.push(elem);
    }
  }
  return matched;
}

  jQuery.each({
    parent: function(elem) {
      var parent = elem.parentNode;
      return parent && parent.nodeType !== 11 ? parent : null;
    },
    parents: function(elem) {
      return dir(elem, "parentNode");
    },
    parentsUntil: function(elem, until) {
      return dir(elem, "parentNode", until);
    }
  }, function(name, fn) {
    ajQuery[name] = function(until, selector) {
      return  fn(until, selector);
    };
  });