This is a sequence number that increments monotonically.
It can be used to track cache updates, or serve as a kind of timestamp for any changing properties.
A special class is used instead of simply an int, so we can elegantly handle such things as wraparound and special cases. There are two special cases. Firstly, a sequence number is 'initial' when it is first created. This sequence is older than any other sequence number. Secondly, a sequence number may be explicitly set to 'old'. This is older than any other sequence number except 'initial'. Finally, we have the explicit number 'fresh', which is newer than any other sequence number. All other sequences are numeric and are monotonically increasing.
Definition at line 43 of file updateSeq.h.