Many systems are organized hierarchically. The CERNDOC documentation system is an example, as is the Unix file system, and the VMS/HELP system. A tree has the practical advantage of giving every node a unique name. However, it does not allow the system to model the real world. For example, in a hierarchical HELP system such as VMS/HELP, one often gets to a lead on a tree such as:
HELP COMPILER SOURCE_FORMAT PRAGMAS DEFAULTS
only to find a reference to another leaf: Please see
HELP COMPILER COMMAND OPTIONS DEFAULTS PRAGMAS
and it is necessary to leave the system and re-enter it. What was needed was a link from one node to another, because in this case the information was not naturally organized into a tree.
Throughout his career, Wilson often answered fan mail and outside requests for his time with this form postcard:
Edmund Wilson regrets that it is impossible for him to: Read manuscripts, write books and articles to order, write forewords or introductions, make statements for publicity purposes, do any kind of editorial work, judge literary contests, give interviews, conduct educational courses, deliver lectures, give talks or make speeches, broadcast or appear on television, take part in writers' congresses, answer questionnaires, contribute to or take part in symposiums or 'panels' of any kind, contribute manuscripts for sales, donate copies of his books to libraries, autograph books for strangers, allow his name to be used on letterheads, supply personal information about himself, supply photographs of himself, supply opinions on literary or other subjects.