For your example, you don't need to match every specific tag - without the anchors (see the 'cozy mystery' line in the above post), 'fantasy' will match both fantasy & high fantasy, for example.
Code:
program:
subjects = $tags;
## Replace this with your collection column
existing_colls = $#kobocoll;
new_colls = strcat(
if '(Comedy|Humor)' inlist subjects then 'Comedy,' fi,
if '(Fantasy|Mythology)' inlist subjects then 'Fantasy,' fi,
if '(Romance|Love)' inlist subjects then 'Romance,' fi,
);
merged_colls = list_union(new_colls, existing_colls, ',');