I'm used to the bog standard COUNT, AVG, MIN, MAX, SUM. It didn't occur to me that there would be others...
wait until you find out you can write your own:
CREATE OR REPLACE FUNCTION agg_leftagg(l TEXT, r anyelement) returns text as $$ BEGIN RETURN CASE WHEN l IS NOT NULL THEN format('%s-%s', l, r) ELSE r::TEXT END; END; $$ LANGUAGE PLPGSQL;