Ok, I'm looking at templates in the manual, 3.2.2, and 3.2.3.
rather than reprint, you can see it here:
http://lilypond.org/doc/v2.6/Documentation/user/out-www/lilypond/Piano-templates.html#Piano-templates
The first one (3.2.2) makes perfect sense to me:
it has \score, and within that it has a voice, and then a pianostaff
in the form
\context PianoStaff <<
\context Staff = upper \upper
\context Staff = lower \lower
>>
That makes sense. Now, 3.2.3 says it will remove the separate vocal
line. I would expect it to look something like
\context PianoStaff <<
\context Staff = upper \upper
\lyricsto upper \new Lyrics \text
\context Staff = lower \lower
>>
or something like that. Instead, you see, in part,
\context Staff = upper {
\context Voice = singer \upper }
\lyricsto "singer" \new Lyrics \text
What really confuses me is that the lower staff is now within its own
<< >>, whereas the upper staff has none of these, only { } . But not
entirely! the \lyricsto section is outside of the upper staff
context's { } !! So, it appears to me that the upper staff has no
syntactic bound!