code: public Interval getIntervalOnlyIfExisting(String aName) {
  		Interval interval = intervalsByName.get(aName);
  		if (interval == null) {
  			throw new UnknownIntervalException(aName);
  		}
  		return interval;
  	}
docs: Same as  getInterval(aName) except that it doesn't create an interval implicitely.
  This method should be used, when you do not want an implicit creation, for example with tieables (thresholds, accumulators).
  @param aName name of the interval.
  @return interval if existing.
  @throws UnknownIntervalException if no such interval exists.