[racket] missing solution 20.2.1 ex:arrows-dd

A candidate for a solution.

Exercise 20.2.1. Explain the following classes of functions:

1. (number  ->  boolean),
2. (boolean symbol  ->  boolean),
3. (number number number  ->  number),
4. (number  ->  (listof number)), and
5. ((listof number)  ->  boolean).

Solution.

  (1) consumes a number and produces a boolean;

  (2) consumes a boolean and a symbol and produces a boolean;

  (3) consumes three numbers and produces a number;

  (4) consumes a number and produces a list of numbers;

  (5) consumes a list of numbers and produces a boolean.