null set

基本解释

When a change is made to a relationship, the related foreign-key property is set to a null value.
对关系进行更新时，相关的外键的属性设置为空值。
If the pszText member is non-NULL, return TRUE to set the item's label to the edited text. Return FALSE to reject the edited text and revert to the original label.
处理这个通知，查找是否有同名项，如果是则返回FALSE，否则将编辑后的文字复制到节点。
However in this case, the link's ref attribute will be set to the value "null," and the link's text content will be hidden unless a flag to show null-link content is set to true.
但是，在这种情况下，这个链接的 ref 属性会设置为 “null” 值，而且链接的文本内容会隐藏起来，除非将显示空链接内容的标志设置为 true。