anchor.diff:  ;; => [42 2 42 4 42]
   ----
   
  -Another interesting traversals include `keys` for focusing on map keys, `vals` 
  +Another interesting traversals include `keys` for focusing on map keys, `vals`
   for focusing on map values, and `indexed` for focusing on index-value pairs.
   
   
   ==== Interoperability
   
  -`cats.labs.lens` implement the `Focus` and `Foci` atom types, which are derived 
  -atoms with either a lens or a traversal. We can focus on a subpart of the 
  +`cats.labs.lens` implement the `Focus` and `Foci` atom types, which are derived
  +atoms with either a lens or a traversal. We can focus on a subpart of the
   structure and change it, reflecting the changes on the original atom:
   
   
  
positive.code: 
  TODO
  ////
  The writer monad comes from Haskell too. It's used for accumulating a log together with the values generated by
  computations. As with the state and reader monads, we don't have to explicitly thread the log manually since
  the monadic bind takes care of that. The type
  ////
  
  
negative.code: what is called a monad comprehension, the difference is that `for` is limited to
  sequences and `mlet` can work with arbitrary monads.
  
  Also, since `mlet` desugars into calls to the Monad's `bind` function, its result
  keeps the type of the monadic values.
  
  
  ==== Lazy sequences
  