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## When and Unless

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.