Kata

In continuation passing style programming, the functions, instead of returning a value, take an extra argument which is a continuation function, taking the value that would have been returned as its argument.

Even the basic operations must take an extra argument. For that end, we defined add(x,y,k) and mult(x,y,k) functions, which should be used in the solution.

Task

Rewrite the following two simple functions in continuation style:

def f(x):
    return x*2

def g(x):
    return 10*x+1

So that, the rewritten functions f and g take an extra parameter, k, and don't return any value, else the tests will fail.
Also, usage of add and mult will be checked.

Finally, use the prepared function show(x,k) in place of print(x) to produce the following code snippet translated into continuation passing style:

def main():
    print( f(g(2)) )

For testing, we introduced the expect_value method which can retrieve the final value to be returned, using the identity function id as the final continuation function.
Usage of id in this kata means the end of the continuation process. (See the example test cases.)