If I want merge in an algorithm where it is often the case that the new map 
has the same keys as the old map, I'd assume using the new map as a 
replacement is faster in those cases.

Of course comparing the keys costs and I'd like to know which is the 
fastest way.

Can I do (= (keys old-map) (keys new-map)) or are the orders not guaranteed?

I see an alternative in (every? new-map (keys old-map)) which would also 
account for cases where the keysets are different but old-map would be 
completely replaced by the merge.

Which one would you recommend, performance wise?

On a side note: I noticed Clojures merge merges a map onto nil by replacing 
nil with an empty map. Wouldn't it be faster if it returned the map right 
away? Should I write a ticket?

Kind regards,
 Leon.

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