--    save the message's value to a specific, unused numeric prefix

--    save the message's value to a new, smallest unused numeric prefix

--      if there's an error, go back to the previous value of the same
--      definition if one exists

--  if a game encounters an error:
--    find the previous version of the definition in the 'head' numeric prefix
--    decrement 'head'

--  if a game encounters a run-time error, send it to the driver and await
--  further instructions. The app will go unresponsive in the meantime, that
--  is expected. To shut it down cleanly, type C-q in the driver.

-- these can be modified live

-- these will be modified live