; "Calling a function is like giving it a hug with parentheses"
  (= __ (square 9))

  ; "Functions are usually defined before they are used"
  (= __ (multiply-by-ten 2))

  ; "But they can also be defined inline"
  (= __ ((fn [n] (* 5 n)) 2))

  ; "Or using an even shorter syntax"
  (= __ (#(* 15 %) 4))

  ; "Even anonymous functions may take multiple arguments"
  (= __ (#(+ %1 %2 %3) 4 5 6))

  ; "Arguments can also be skipped"
  (= __ (#(* 15 %2) 1 2))

  ; "One function can beget another"
  (= 9 (((fn [] ___)) 4 5))

  ; "Functions can also take other functions as input"
  (= 20 ((fn [f] (f 4 5))
           ___))

  ; "Higher-order functions take function arguments"
  (= 25 (___
          (fn [n] (* n n))))

  ; "But they are often better written using the names of functions"
  (= 25 (___ square))