将元素定义为xml架构中另一个元素的列表(define an element as a list of another element in xml schema)

我想为社交网络网站定义一个简单的XML Schema。我已经定义了一个Person元素及其相关属性和其他子元素。它是我定义的模式的一部分。 ：

现在我想为Person定义另一个孩子，因为它是朋友，但我不知道我怎么能这样做。任何人都有任何想法吗？

I want to define a smaple XML Schema for a social network website.I've defined a Person element and it's related attributes and other child elements.it's a part of the schema which I've defined. :

now I want to define another child for Person as it's friends but I dont know how can I do this.does any body have any Idea about this?