When there's a rest argument, it's an improper list. The `formalss` here is from this part of the expansion of keyword lambdas:

````
(case-lambda
  ((given-kws given-args . rst)
   (#%app unpack8 given-kws given-args rst)))
````

The rest argument only appears to show up in the branch of the `case-lambda` with all the arguments, hence the `+inf.0` in this code.