4. The method of claim 3 , wherein the extra information comprises the duration of the document being opened, the number of times that the same document is opened by the user, the time when the selected document is opened, and/or the opening sequence in which the user selected document is opened.
3. The method of claim 1 , further comprising: detecting and storing extra information of the related document list, and recording the document relationship according to the related document list and the extra information.