	!a	abort current file transmission
	!c	close current receiving file
	!e	echo terminal input locally (i.e. in Unix)
	!f	files in use (displays name of receiving file and/or
			transmitting file, if any)
	!h	help (displays this summary)
	!n	no echo locally
	!q	quit the link program
	!r	receive into file (all displayed characters are written
			to the specified file until a subsequent 'close')
	!s	send contents of file (without mappings or synchronisation)
	!t	transfer text file (e.g. to Maximop) line-by-line, mapping
			line terminators and waiting for prompts between lines 
	!u	temporarily return to Unix (until a subsequent ^D)
			link remains active and received characters
			continue to be displayed.
	!w	wakeup TIP (sends a sequence of delete codes)
