public class Stack
 {
  Entry top;
  public void Push(object data) {
   top = new Entry(top, data);
  }
  public object Pop() {
   if (top == null) throw new InvalidOperationException();
   object result = top.data;
   top = top.next;
   return result;
  }
  class Entry
  {
   public Entry next;
   public object data;
   public Entry(Entry next, object data) {
    this.next = next;
    this.data = data;
   }
  }
 }
为什么一个类可以嵌套另一个类，这些代码什么意思？我看不懂，有谁给我说说，谢谢了。