public int numSlots() { return numSlots; }
// returns the next coarser granularity.
// FULL -> 5m -> 20m -> 60m -> 240m -> 1440m -> explosion.
public Granularity coarser() throws GranularityException {
if (this == LAST) throw new GranularityException("Nothing coarser than " + name());
return granularities[index + 1];
}