* There is no clear reason why the map should hold const std::strings
* This class is never derived, there doesn't seem to be a compelling
  reason to ever do so, so no need to make virtual members
* In general const member data is an anti-pattern, the *only* reason
  is to prevent assignability, and the accepted idiom for that is to
  to declare the assigment operator private. This change does so, and
  also prevents copy construction.