He forgot `group-by` which is very useful. (group-by :age [{:name "joe" :age 13 :sex \m} {:name "sara" :age 13 :sex \f}

He forgot `group-by` which is very useful. (group-by :age [{:name "joe" :age 13 :sex \m} {:name "sara" :age 13 :sex \f} {:name "John" :age 20 :sex \m}]) => {13 [{:age 13, :sex \m, :name "joe"} {:age 13, :sex \f, :name "sara"}], 20 [{:age 20, :sex \m, :name "John"}]}