Insert mode causes all characters typed to be inserted into the text area,
at the position indicated by the cursor.

Insert mode is left by one of four events:
   - Entering CTRL_A (change to OVERSTRIKE 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 insert mode, function keys, immediate commands and interrupt commands
remain available.