Submission instructions:

Submit the solutions in a single .ss file. Task 2 should
be submitted as a Scheme comment.

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1. Solve Exercise 4.3.1 in the textbook, that is, add the if0
construct to all relevant definitions of F1WAE *including the parser*.

Add a testcase (using the "test" function) which includes a definition
and usage of the factorial function. Feel free to add more language
primitives (such as "*") to the language, if you think this is necessary.

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2. It is unsatisfactory to have many language primitives (such as if0).
It would be more elegant if there would just be a number of "pre-installed"
functions that are called using normal function application.

Let's say you define a function "myif0" as a F1WAE-function of three parameters
(ignore the problem that our functions can only have one parameter). The parameters
are "if-part", "then-part" and "else-part", and (myif0 if-part then-part else-part)
is just implemented as (if0 if-part then-part else-part).

Think about whether you could use "myif0" rather than the builtin "if0" in your
factorial function. Explain why or why not. If not, think about a possible fix
that would let you use "myif0".