Overstrike mode causes all characters typed to replace the ones in the
text area, at the position indicated by the cursor. When the cursor is at
a RETURN-character, characters are inserted instead. This prevents lines
from being joined unexpectedly in overstrike mode. The RETURN-character
itself cannot be used to separate lines. In overstrike mode, its function
is only to move to the beginning of the next line. Similar respect for the
end of a line is present in the functioning of the BACKSPACE and DEL keys.

Overstrike mode is left by one of four events:
   - Entering CTRL_A (change to INSERT mode)
   - Entering CTRL_G (change to COMMAND mode)
   - Entering ESC (change to COMMAND mode)
   - Pressing a function key containing the CTRL_X-code for changing the
     mode.

In overstrike mode, function keys, immediate commands and
interrupt commands remain available.