Creates a guard for a limit in parts per thousand, ready to transmit at once.
10 is 1%; 0 forbids transmitting and 1000 or more imposes no silence.
The earliest time the next transmission may start, in microseconds on the caller's
clock, or null when the limit forbids transmitting.
The limit the guard enforces, in parts per thousand.
Whether a transmission may start at a time in microseconds on the caller's clock.
Records a transmission and the silence it owes, returning its airtime in microseconds.
How long the radio must still stay silent, in microseconds, or null when the
limit forbids transmitting.
The silence a radio owes after its transmissions under a duty-cycle limit.