1.DBG> SET EDITOR '@MAIL$EDIT ""'

      This command causes the EDIT command to spawn the command line
      '@MAIL$EDIT ""', which starts the same editor as you use in
      MAIL.

    2.DBG> SET EDITOR/CALLABLE_TPU

      This command causes the EDIT command to start callable DECTPU
      with the default command line of TPU.

    3.DBG> SET EDITOR/CALLABLE_TPU TPU/SECTION=MYSECINI.TPU$SECTION

      This command causes the EDIT command to start callable DECTPU
      with the command line TPU/SECTION=MYSECINI.TPU$SECTION.

    4.DBG> SET EDITOR/CALLABLE_EDT/START_POSITION

      This command causes the EDIT command to start callable EDT
      with the default command line of EDT. Also the /START_POSITION
      qualifier is appended to the command line, so that the editing
      session starts on the source line that is centered in the
      debugger's current source display.