\song{[% purpose %]}{O come, o come Emmanuel}{%
    [% IF parts != '0' -%]
    \lilypondfile{library/freie_lieder/veni_immanuel/o_come_o_come_emmanuel_leadsheet.ly}%
    [% ELSE -%]
    \lilypondfile{library/freie_lieder/veni_immanuel/o_come_o_come_emmanuel_lz.ly}%
    [% END -%]
}