- the indices(const wordRes& allow, const wordRes& deny) methods
  were not defined consistently with the wordRes::filter handling.

  wordRes::filter - allow/deny both empty:
      = no filtering (ie, accept everything)

  indices(allow,deny) - allow/deny both empty:
     = return identity list (NEW behaviour)
     = return empty list (OLD behaviour)

  Consider the old behaviour a minor bug since the limited number of
  callers had their own checks that avoided the poor behaviour.

  Example of the inconsistent behaviour:

     pbm.indices(wordRes(), wordRes({"non-existent"}))
        --> identity list (old, new behaviour)

     pbm.indices(wordRes(), wordRes())
         --> identity list (new behaviour)
         --> empty list    (old behaviour)