- can use 'XX.empty()' instead of 'XX.size() == 0', 'XX.size() < 1' or
'XX.size() <= 0' or for simpler coding.
It also has the same number of characters as '!XX.size()' and /might/ be
more readable
- many size checking had 'XX.size() > 0', 'XX.size() != 0', or 'XX.size() >= 1'
when a simple 'XX.size()' suffices
- operator+= : add in the listed keys
- operator-= : remove the listed keys
- operator&= : intersection of keys
- added xfer constructor (just in case)
- moved labelHashSet typedef to HashSet.H, for consistency with the
wordHashSet typedef being there and since it is used so often
