opus and piece fields in top level header.

[Richard Shann]

In the 2.18 it does not seem to mention that the opus and piece fields
are not output anywhere if they are set in a \header{} block *outside*
any \score{} block, but this seems to be the case.
It is not a problem, but I would like to be sure this is intended to be
the case as I have finally got round to creating a sane interface to the
titles from Denemo and I see that they should be available for setting
at the top level as they don't (?) do anything there.
If there is someone knowledgeable that can confirm this I'd be grateful

Richard