how to reuse common header attributes

[Xavier Noria]

Hi,

I am getting familiar with the docs, but can't yet figure this out by myself.

I am transcribing a song in C, and would like the same document to
include its transposition in E flat. The file structure I have come
with so far is:

    \version "2.16.1"

    \include "english.ly"

    melody = \relative c'' { ... }
    \addlyrics { ... }

    \bookpart {
      \header {
        title    = "title"
        subtitle = "(Transcription in C)"
        composer = "composer"
        tagline  = "tagline"
      }

      \score {
        { \melody }
      }
    }

    \bookpart {
      \header {
        title    = "title"
        subtitle = "(Transcription in E flat)"
        composer = "composer"
        tagline  = "tagline"
      }

      \score {
        { \transpose ds c \melody } % "ds" because "ef" yields a weird key
      }
    }

As you see some data repetition remains in the headers (title,
composer, and tagline, subtitle differs).

Questions:

1) The structure and variable usage are idiomatic?

2) Is there a clean way to factor those common header attributes out somehow?