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The when and unless forms conditionally
evaluate a single body of expressions:
- (when test expr ) evaluates the
expr body expressions only when test returns a true
value.
- (unless test expr ) evaluates the
expr body expressions only when test returns #f.
The result of a when or unless expression is the result of
the last body expression if the body is evaluated, or void (see
section 4.1) if the body is not evaluated.
PLT