rate( frequency )
Halts computations until 1.0/frequency seconds after the previous
call to rate().
For example, rate(50) will
halt computations long enough to make sure that at least 1.0/50.0 second
has elapsed (if this much time has already elapsed, no halt is performed).
If you place rate(50) inside a computational
loop, the loop will execute at a maximum of 50 times per second, even if
the computer can run faster than this. This makes animations look about
the same on computers of different speeds, as long as the computers are
capable of carrying out 50 computations per second.
