如何找到列表列表中的公共元素？

我想弄清楚如何比较n个列表来找到共同的元素。
例如：

I'm trying to figure out how to compare an n number of lists to find the common elements. For example:

p=[ [1,2,3],
    [1,9,9],
      ..
      ..
    [1,2,4]

>> print common(p)
>> [1]

现在如果我知道元素的数量，我可以做比较：

Now if I know the number of elements I can do comparions like:

for a in b:
  for c in d:
    for x in y:
...

但是如果我不知道有多少元素p，它不工作。我看过这个比较两个列表的解决方案
http://*.com/a/1388864/1320800

but that wont work if I don't know how many elements p has. I've looked at this solution that compares two lists http://*.com/a/1388864/1320800

但是花了4个小时试图找到一个递归的方法，一个解决方案仍然逃避我，所以任何帮助将高度赞赏！

but after spending 4 hrs trying to figure a way to make that recursive, a solution still eludes me so any help would be highly appreciated!