[sciurius]

An interesting use of it would be to show a different field if one didn't exist, i.e. %ARTIST: -ARTIST, %COMPOSER%%

I'm not quite sure I understand the semantics of this construct.

My intention was:

If ARTIST has a value, substitute everything after the :, repeat. Probably I should have written

%ARTIST: - %ARTIST%, %

or, more formally,

%<field>:<text if defined>%

This would be fairly easy: on 1st pass, check if artist has a value, if so replace by everything after the colon. So it becomes

- %ARTIST%,

On second pass, there's no colon anymore so the field value can be substituted.

For an if-then-else functionality, we could do something like

%<field>:<text if defined>|<text if undefined>%

This looks simple, elegant, easy to implement, yet powerful.