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 RDINFO accesses blocks of text in what computer science calls a "tree".
 A "tree" is simply a block of  text which points to other blocks,  like
 the root of a tree is connected to the branches.  These new blocks  can
 point  to  other  blocks,  like  big  branches  splitting  into  little
 branches, and so on.  The text blocks are called "nodes".

 RDINFO does not care if the branches connect to branches at other  than
 the current level, so it  actually handles "directed graphs"; all  that
 matters is  that  some  nodes  point to  others.

 Computer trees are  conventionally written  with the root  at the  top,
 like an upside-down tree, because  we like to start  at the top of  the
 page and move down.  A simple INFO tree might look something like this:

           |                        |                        |
          One                      Two                     Three
           |                        | 
   -----------------                | 
   |       |       |                |                      
 One-a   One-b   One-c            Two-a
                                |        |
                              Two-b    Two-c

 The "M"  (menu) command  in node  "Top" can  take you  to nodes  "One",
 "Two", or "Three".  In "One", the "M" command can then get you to nodes
 "One-a", "One-b", and "One-c"; in "Two", it can get you to "Two-b"  and
 "Two-c".  Node "Three" does not have a menu; it is a "leaf" node.

 The "U" command can take you from "Two-b" up to "Two", and another "U"
 gets you up to "Top".

 The "N" (next) command takes you from "One-a" to "One-b", another  gets
 you to "One-c".  A third  might go anywhere else  in the tree, but  the
 current  implementation  of  RDINFO   automatically  figures  out   the
 connections,  and  "One-c"  is  not  given  a  "next"  node.   The  "P"
 (previous) command steps in the reverse direction to "N".

 The "G" (go to) command can  take you immediately anywhere you want  to
 go in the INFO tree, if you know the node name.

 The "L" (last) command takes you back to the last node you visited,  so
 you need not remember how you got to the current one.

 The nodes in  the tree do  not have to  reside in the  same text  file,
 because node names can contain parenthesized filenames.  This lets  the
 documentation in  the  INFO  system  grow quite  large  and  have  many
 authors, without  having to  have enormous  text files  or worry  about
 author  conflicts  and  file  write  access  problems.   Usually,  text
 relating to a single subject will be  kept in one file, because it  can
 be then  easily  searched  with  the "S"  (search)  and  "K"  (keyword)

 Moving around the INFO tree is quite fast, because each INFO file is  a
 direct access file with  a node directory at  the beginning which  lets
 RDINFO position to any node in any order, without having to read all of
 the file.