using if variable in lilypond

[Stjepan Horvat]

hi guys..
i would want to define my font sizes in the next order:

\header {
        title = \markup { \fontsize #+4   "BEI MIR BIST DU SCHÖN"}
        subtitle =      \markup { \fontsize #+1   "(Yiddish: Bay mir bistu 
sheyn)"      }
        poet =  \markup { \fontsize #+1   "Jacob Jacobs"        }
        composer =      \markup { \fontsize #+1   "Sholom Secunda"}
}

so i would want to make a config out of it so i woudn't always have to
add the extra text..
i used something like that in external config.ly:

title = \markup { \fontsize #+4 \title }
and so on..

but that only works if i include the config file after the header and
if all sub header name exist..

my question:
is there a posibility to make something like that.

if ( title exists ):
  title = \markup { \fontsize #+4 \title }

thanks..:)
p.s. is there an way to learn that stuff (sheme) without always asking
you guys..?!
i'm familiar with bash, perl and python..is it posible to integrate
thouse lenguage into lilypond?!


-- 
Nesmotren govori kao da mačem probada, a jezik je mudrih iscjeljenje.
Izreke 12:18