(def m
    {
        :u1 { :p1 2 :p2 3}
        :u3 { :p2 4 :p3 7}
        }
    )
=> (var leintest.user-user-recommander.user-user-recommander/m)
(merge-with + m {:u5 {:p3 5}})
=> {:u1 {:p1 2, :p2 3}, :u3 {:p3 7, :p2 4}, :u5 {:p3 5}}
(merge-with + m {:u1 {:p1 5}})
ClassCastException clojure.lang.PersistentHashMap cannot be cast to java.lang.Number  clojure.lang.Numbers.add (Numbers.java:126)