anchor.diff:  ;; => 4
   ----
   
  -NOTE: they are analogous to the `then` and `then'` but that accepts multiple
  +NOTE: these are analogous to `then` and `then'` but accept multiple
   transformation functions.
   
   If you want to handle rejected and resolved callabacks in one unique callback,
  -then you can use `handler` chain function:
  +then you can use the `handler` chain function:
   
   [source, clojure]
   ----
  
positive.code: this might cause unexpected latency. It may block other threads in the thread
  pool from doing other, maybe more important, tasks.
  
  For such cases, **promesa** exposes an additional arity for provide a
  user-defined executor to control where the chained callbacks are executed:
  
  [source, clojure]
  
negative.code: === Creating a promise
  
  There are several different ways to create a promise instance. If you just want
  to create a promise with a plain value, you can use the polymorphic `promise`
  function:
  
  [source, clojure]
  