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: 
  [source, clojure]
  ----
  (foldr (fn [v wc] (+ v wc)) 0 [1 2 3 4 5])
  ;; => 15
  ----
  
  
negative.code: 
  [source, clojure]
  ----
  (m/fapply (make-greeter "es") (maybe/just "Alex"))
  ;; => #<Just "Hola Alex">
  
  (m/fapply (make-greeter "en") (maybe/just "Alex"))
  ;; => #<Just "Hello Alex">
  
  (m/fapply (make-greeter "it") (maybe/just "Alex"))
  ;; => #<Nothing>
  ----
  
  