anchor.diff:  (m/fmap inc [1 2 3 4])
   ;; => [2 3 4 5]
   ----
  +
   
   ==== nil
   
  
positive.code: ;; => (4 5 6 5 6 7)
  ----
  
  ==== Set
  
  Sets implement almost every abstraction in _cats_, from Semigroup to Monad.
  
negative.code:         (fn [v] #{v (inc v)}))
  ;; => #{1 4 3 2}
  ----
  
  
  ==== Map
  
  