problem 2.9.4

[lishichengyan]

Thank you for your work!

Just found your problem 2.9.4 is missing, I think the answer should be:

f(x, y) => 
f(x-1, x*y) => 
f(x-2, (x-1)*x*y) => 
f(x-3, (x-2)*(x-1)*x*y) => ... => 
f(1, 2*3*...*(x-2)*(x-1)*x*y) =>
f(0, 2*3*...*(x-2)*(x-1)*x*y) = (x!)*y

mathematically it's the factorial of x times y

[TorbenRahbekKoch]

You're welcome!

I obviously haven't looked at this for a very, very long time :)

You are more than welcome to create a Pull request, remember to include a test, as well!