Exercise 9 of the Narrowing section asked you to code a Curry program that generates all (non-deterministically) the well-formed expressions of a Game of 24 problem.  To remind you, a Game of 24 problem is a set of 4 digits.  The program of exercise 9 should generate any expression that combines, in any possible way, the digits with the 4 arithmetic operations.

Now, you are required to complete the program.  Instead of producing any well-formed expression, your program should produce the subset of these expressions that evaluate to 24.