Accessibility of score/bookpart header variables in headerMarkup

[Leah Velleman]

> Not top-posting

There are two related things I want to mention here — one is
an inconsistency between program behavior and documentation,
and one is a feature request.

First, the inconsistency: The manual
(http://www.lilypond.org/doc/v2.19/Documentation/notation/
creating-titles-headers-and-footers#titles-explained) says
that evenHeaderMarkup and other header/footer variables...

> can only access text fields from top-level \header blocks
> (which apply to all scores in the book)

In fact, this is not true. they can also access text fields
from \header blocks at levels as low as bookpart, as the
following MWE shows:

\version "2.19"
\paper {
  oddHeaderMarkup = \markup { \fromproperty #'header:title }
  evenHeaderMarkup = \markup { \fromproperty #'header:title }
  scoreTitleMarkup = \markup { }
  bookTitleMarkup = \markup { }
}

\book {
  \bookpart {
    \header { title = "foo bar baz" }
    \score {
      \new Staff {  c d e f g a b c }
     }
  }
}

Second, the feature request: It would sometimes be useful to
be able to access fields from score-level header blocks as
well as from top-, book- and bookpart-level ones. (The use
case I have in mind is printing "PIECE TITLE (cont'd)" at the
top of the second and subsequent pages of a piece.  There are
probably others.) The restriction to bookpart-level and
higher header blocks seems arbitrary, and I wonder if it
could be removed.

Best,
Leah