anchor.diff:  ----
   
   The returned function form `adder` is a closure.
  -This means, it can access all of the variables that we're in scope when the function was created.
  +This means, it can access all of the variables that were in scope when the function was created.
   `add-five` has access to `x` even though it is outside of the `adder` function definition.
   
   === Filter
  
positive.code: ----
  
  This code works fine, but it's bulky and confusing.
  We can simplpify this using `filter`, a higher order function.
  
  [source, clojure]
  ----
  
negative.code: 
  It's common to see function definitions in Clojure using `defn` like `(defn foo ...)`.
  However, this is just syntactic sugar for `(def foo (fn ...))`
  `fn` returns a function object.
  `defn` returns a var which points to a function object.
  
  == Higher Order Functions
  
  