I need help with Calibre device templates

I'm basically trying to have two folder structures based on whether or not a series is defined. For example:

If Series exists, I want to use this structure

Series/{series}/{series_index} - {title}

otherwise I want to use this one

Books/{author_sort}/{title}

Any idea how I can achieve this?



I almost solved the problem but this isn't quite the same structure:
{series:'ifempty($, 'Books')'}/{author_sort}/{series_index} {title}