SET(DEFAULT_DIRECTORY)
     Determines the directory that will be used as your current, default
     directory.
  Syntax
     [old_default_string :=] SET (DEFAULT_DIRECTORY, new_default_string)
  Parameters
     DEFAULT_DIRECTORY    A keyword indicating that the SET built-in is
                          being used to control which directory is used as
                          your current, default directory.
     new_default_string   A quoted string naming the directory to which you
                          want the default changed.
     old_default_string   Optionally, the string naming the old default
                          directory.
  Comments
     Note that when exit from DECTPU, your current, default directory is not
     restored to the default that was set when you invoked DECTPU.
     Note, too, that if you issue the EVE command DCL SHOW DEFAULT, the
     directory shown is not always the new default directory, even though
     the setting has actually been changed.  To update DCL's tracking of
     your current, default directory, you can use the EVE command DCL SET
     DEFAULT.
     If you specify a logical name that is a search list, DECTPU will set
     your current, default directory to the first directory in the search
     list.
  Example
     prev_dir := SET (DEFAULT_DIRECTORY, "DISK1:[WALSH.PINK]");
     This statement sets your current, default directory to [WALSH.PINK] on
     the device DISK1.  The variable "prev_dir" contains the string naming
     the previous default directory.
  Related Topics
     GET_INFO(SYSTEM)