code: public T get(Instant effectivAt) {
  		Entry<Instant, T> entry = values.floorEntry(effectivAt);
  		if (entry == null) return null;
  		return entry.getValue();
  	}
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.