That doesn't really address what either myself or Mitch were talking about, but both newspaper and book can selectively enable/disable their derivatives--check their admin pages; therefore, this is introducing a change in functionality if we don't check both properly.

I was addressing Mitch's concerns about attempting to call functions which may not exist, due to the modules not being enabled... They should in fact be soft dependencies (not in the .info). What we need is another function which obtains all enabled derivatives, but filters out those for which we do not have the modules enabled...  This issue appears to have been overlooked a couple of times.