anchor.diff:  (m/mlet [a (maybe/just 1)
            :when (= a 2)]
      (m/return (* a 2)))
  -;; => #<Nothing >
  +;; => #<Nothing>
   ----
   
   Any monadic type that implements `MonadZero` can be combined with guards inside `mlet` bindings. Here is an example with
  
positive.code:                     (m/mzero))
                    (fn [_]
                      (m/return (* a 2))))))
  ;; => #<Nothing >
  ----
  
  As you can see in the above example the predicate `(= a 2)` returns either a monadic value `(m/return nil)` or the identity value
  
negative.code: [source, clojure]
  ----
  (m/fmap inc (nothing))
  ;; => #<Nothing>
  ----
  
  Oh, awesome, instead of raising a `NullPointerException`, it just returns *nothing*. Another
  