Nondefinable Keys

  You can define all keys on the LK201 and VT100-series keyboards -- except
  the following:

     BREAK        COMPOSE CHARACTER   CTRL (by itself)   LOCK or CAPS LOCK
     HOLD SCREEN  SET-UP              SHIFT

  On the LK401 keyboard you also cannot define the ALT FUNCTION key.

  While TPU does not prevent you from defining keys F1 through F5 or the
  ESCAPE key, the code bound to these keys cannot be executed.

  The PF1 key is the default TPU shift key.  You cannot define PF1 unless
  you specify a different shift key by using SET(SHIFT_KEY...).

  You can define and execute the following keys using the DECwindows
  interface.  You can also define the keys on character-cell terminals --
  although doing so is not recommended because you can execute the keys only
  under special terminal settings, as follows:

     CTRL/C, CTRL/O,         To execute a procedure or program bound to
     CTRL/X, and F6          one of these keys on a character-cell terminal,
                             you must have entered the DCL command
                             SET TERMINAL/PASTHRU.

     CTRL/T and CTRL/Y       To execute a procedure or program bound to
                             either of these keys on a character-cell
                             terminal, you must have entered the DCL command
                             SET TERMINAL/PASTHRU or SET TERMINAL/NOCONTROL.

     CTRL/Q and CTRL/S       To execute a procedure or program bound to
                             either of these keys on a character-cell
                             terminal, you must have entered the DCL command
                             SET TERMINAL/NOTTSYNC.

  Defining CTRL/M or RETURN affects the other as well.  Similarly, defining
  CTRL/I or TAB affects the other as well.

  The EVE editor has the same restrictions as TPU, except EVE does not
  let you redefine RETURN, does not let you define typing keys (unless used
  with a modifier such as GOLD or CTRL), and lets you redefine the DO key
  only if you have defined another key as DO.  Also, EVE does not have a
  default shift key (or GOLD key), so PF1 can be defined.  (For more
  information, see EVE help on SET GOLD KEY.)

  Related topics

     DEFINE_KEY   KEY_NAME   KEYNAMES TABLE   SHIFT_KEY