[chaley]

Originally Posted by ownedbycats

I managed to figure this, perhaps a bit clumsily -- also changed some of the reference names to be a bit clearer

program:
...
	if new_colls == '' then existing_colls = $#kobocoll fi;
	if new_colls then existing_colls = list_difference($#kobocoll, $#booktype, ',') fi;

	if new_colls == '' then new_colls = strcat($#booktype, ',') fi;

	merged_colls = list_union(new_colls, existing_colls, ',');

explanation:

new_colls generates a set of new collections with the strcat

existing_colls is taken from the existing #kobocoll value. If new_colls exists, it removes booktype from the list

If new_colls doesn't exist, it sets it to #booktype

merged_colls combines new_colls and existing_colls.

FWIW: this is a simplified version of what you wrote:

program:
...
	if !new_colls then 
		existing_colls = $#kobocoll;
		new_colls = $#booktype
	else
		existing_colls = list_difference($#kobocoll, $#booktype, ',')
	fi;
	merged_colls = list_union(new_colls, existing_colls, ',');