I wanted to display links the details pane depending on which ids were available. Here's the original template before I realized it didn't work as intended:
Code:
program:
first_non_empty(
if $publisher=='Fanfiction.net' then id = select($identifiers, 'ffnet') fi,
if $publisher=='Archive of Our Own' then id = select($identifiers, 'ao3') fi,
id = select($identifiers, 'goodreads'),
id = select($identifiers, 'ff'),
id = select($identifiers, 'google'),
id = select($identifiers, 'url')
)