Returns a map containing all the keys that are in the domains of both
map1 and map2, where the value for each key is the result of calling
val-fn on the value from map1 and the value from map2. val-fn
defaults to simply returning its second argument, so the entries in map2
simply shadow those in map1. The default for the new map is the result
of calling val-fn on the defaults for the two maps (so be sure it can
take these values).