merge-with

(merge-with f maps...)

Answers one fresh map over every map's pairs, conflicts resolved through f over the old and new values. nil maps contribute nothing, and a Java Map its entries (interpreter and JVM); (merge-with f) is nil. The result keeps a record's type only when the first map is one, like the oracle: a nil first map answers a plain map even when a later map is a record. As a value the function, then any number of maps, with the same record rewrap.