;; 'merge-with' works with an arbitrary number of maps:
user=> (merge-with +
{:a 1 :b 2}
{:a 9 :b 98 :c 0}
{:a 10 :b 100 :c 10}
{:a 5}
{:c 5 :d 42})
{:d 42, :c 15, :a 25, :b 200}