clojure.set/join

(clojure.set/join xrel yrel) (clojure.set/join xrel yrel km)

Answers the relational join: each pair of members that agree on the keys the two relations' first members share -- or, with km, on the xrel keys km maps to yrel keys -- merged into one map. The smaller relation is indexed; with no key in common every pair joins, and an empty relation answers #{}. Reached also as alias/var or a referred bare join; works as a function value too.