anchor.diff:  .Example using fmap over *nothing*.
   [source, clojure]
   ----
  -(m/fmap inc (nothing))
  +(require '[cats.monad.maybe :as maybe])
  +(require '[cats.core :as m])
  +
  +(m/fmap inc (maybe/nothing))
   ;; => #<Nothing>
   ----
   
  
positive.code: ----
  (require '[cats.monad.maybe :as maybe])
  
  (pure maybe/context 5)
  ;; => #<Just 5>
  ----
  
  
negative.code: 
  [source, clojure]
  ----
  (m/foldr (fn [v wc] (+ v wc)) 0 [1 2 3 4 5])
  ;; => 15
  ----
  
  