[eschwartz]

http://manual.calibre-ebook.com/temp...-not-found-val

str_in_list(val, separator, string, found_val, not_found_val)

str_in_list(val, separator, string, found_val, not_found_val) – treat val as a list of items separated by separator, comparing the string against each value in the list. If the string matches a value, return found_val, otherwise return not_found_val. If the string contains separators, then it is also treated as a list and each value is checked.

use "," as separator, val will be field('tags') and you want to search for string "First", then return found_val "First". not_found_val should return empty "" which first_non_empty should skip over, I think.

Personally, I think you should use general program mode for this:

program:
	first_non_empty(
		str_in_list(
			field('tags'),
			",",
			"First",
			"First",
			""
		),
		field('series'),
		"Uncategorized"
	)