code: public T get(Instant effectivAt) {
  		Entry<Instant, ValueListEntry<T>> floorEntry = values.floorEntry(effectivAt);
  		if (floorEntry == null) return null;
  		ValueListEntry<T> vle = floorEntry.getValue();
  		if (!(effectivAt.isBefore(vle.expires))) return null;
  		return vle.value;
  	}
docs: Gets the value effective at the specified instant in time.
  
  Returns null if the property had no value at the specified time.
  
  @param effectivAt
  @return the value of the property at the specified time.