technique for mixing / matching tunes

[Jason Addison]

What is the recommended technique for defining tunes in separate files
so that they can be used to generate pdf with only that tune, or pdf
wth several consecutive tunes? For example (what I'm try and failing
with--I thing because I use variables outside of score but inside of
book):

<caution: approximate/brief lilypond syntax below>

tune_a.ly:
parta = { ... }
partb = { ... }
score { parta partb headerinfo }

tune_b.ly
parta = { ... }
partb = { ... }
score { parta partb headerinfo }

tune_a-single.ly: <generated on the fly with make/sh>
include single_tune_hdr.ly
include tune_a.ly

set_one.ly:
include set_hdr.ly
book {
include tune_a.ly
include tune_b.ly
}

Everything works fine except for set_one.ly. I tried putting the
variable defs inside score but that didn't work. It works if I don't
use variable, but I rely heavily on using variables. Any ideas?

Thanks,
Jason