Command mode is the normal mode, enabling command input. All characters
typed are echoed in the command input. Two ESC-characters function as
terminator. As soon as this double-escape is entered, the command-line is
executed as a command sequence. When it finishes, TV is ready for a new
command.

A sequence finishes because either its last command terminates, or an error
occurs anywhere in the command sequence. After an error, processing stops
immediately. The corresponding error-message is displayed in the display
area.

Command mode is left by one of two events:
   - Entering CTRL_A as an immediate command (change to OVERSTRIKE mode)
   - Executing a command containing the CTRL_X-code for changing the
     mode.