The following library routines are available ON ITS ONLY:

	Buffered I/O:
		Input: there is a default prompt string (settable), which is
	printed when ^L in typed; if the default prompt string is not set,
	then any partial (buffered) output line is used; delete deletes a
	character as usual; CR completes the reading of a buffered line.
		Output: nothing happens until the buffer is full, a CR is
			sent, or a tyo_flush is done.

		char tyi (); reads one buffered character; CR is changed to LF
		tyo (c)  char c; outputs a buffered char; ^P changed to ^
			followed by P; full buffer or CR causes buffer to be
			sent
		tyo_flush (); forces buffer to be sent
		tyos (s)  char *s; calls tyo repeatedly, changing CR to LF
		setprompt (s)  char *s;  sets the default prompt string

	Unbuffered I/O:
		char utyi ();  flushes the output buffer and reads a char;
			no mapping or echoing performed
		utyo (c)  char c;  flushes the output buffer and send the
			char; no mapping done EXCEPT ^P changed to ^ followed
			by P
		spctty (c)  char c;  flushes the output buffer then sends
			^P followed by the argument; for ITS display codes.

	Interrupts:
		^G and ^S are set to interrupt; they result in signals
	of ctrlg_interrupt and ctrls_interrupt, respectively.  To
	associate a routine with them (one which does nothing is a good
	way to ignore the interrupts) you do:
		on (ctrlg_interrupt, f)   (likewise for ^S)
	where f is a function taking no arguments and returning no
	results.  The interrupt character will have been read at
	interrupt level.  If you want to handle TTY interrupts yourself,
	you can do
		on (ttyi_interrupt, fn) and fn will be called on a
	tty input interrupt; ityic (tty_input_channel) will return
	an interrupt char (-1 means it went away); tty_input_channel
	is an int defined in one of the library routines.  To set
	which chars will interrupt, you can use:
		ttyget (tty_input_channel, block)
	and	ttyset (tty_input_channel, block)
	where block is an array of 4 ints; these 4 words are the
	results and the arguments of the corresponding ITS .call's,
	in the same order.  Initially all chars activate, and ^G and ^S
	interrupt.