flash routines are used to alert the
terminal user. The routine beep sounds an audible alarm on the
terminal, if possible; otherwise it flashes the screen (visible bell).
The routine flash flashes the screen, and if that is not possible,
sounds the alert. If neither alert is possible, nothing happens.
Nearly all terminals have an audible alert (bell or beep), but only
some can flash the screen.
These routines return
#t if they succeed in beeping or