Map merging algorithms. Enables specification of merging function on a per key basis.
Merge function that dispatches on the map entry key
Returns a map that consists of the rest of the maps conj-ed onto the first. If a key occurs in more than one map, the mapping(s) from the latter (left-to-right) will be combined with the mapping in the result by calling: (merge-key key-algorithms key val-in-result val-in-latter).