[racket] Testing "impossible" branches in the code

Hi,

I've spent some time on the following problem, and would like to ask for your help:

I'm writing a critical piece of an application, and would like to ascertain in a "belt and suspenders" kind of way that I get a customized exception if something goes wrong.

The approach I have used is based on independent checks (conditionals) that might trigger the exception. The problem is that I have not found an elegant way to test this code, since there should not be such combinations of parameters that could lead to this branch in the conditional. One way would be to extract these pieces of code to auxiliary functions and call these from the tests with those "impossible" parameters that would trigger the exception. However, this would split the code into ugly, detached pieces.

Simplified example:

(let ((a 2)(b 2))
  (if (= (+ a b) 4)
      #t
      (error "impossible!")))

Is there some practical way to access the variable bindings inside a function from tests (similarly to the "set! <var>" pop-up menu in the debugger window) in order to inject faults to trigger these exceptions?

Best,

Mikko T.