Command history

	There is a crude command history feature in the calculator.
	If a terminal line begins with an exclamation mark or back quote
	character (! or `), then the terminal line is handled specially.

	The command just typed can be re-executed by typing the
	line '``' (two back quotes).

	Other previous commands can be executed again by typing '`nn'
	(backquote <number>), where nn is the command number to be 
	ex-executed.  This number is displayed in the prompt for each 
	input line, so it is easy to re-execute commands that are still 
	visible on the screen.  Negative numbers can be used to re-execute 
	the n'th command back.

	The list of 20 previous commands can be displayed by typing '`h'
	(backquote h).  By typing '`hnn' (backquote h <number>), one
	can ex-execute the last 'nn' commands.

	Up to 255 commands are saved in the history stack.

	A UNIX command can be executed by typing '!cmd', where cmd
	is the command to execute.  If cmd is not given, then a shell
	command level is started.
