There are some counter-intuitive discrepancies between HTTP and the final Sexp:

> If a checkbox is unchecked when its form is submitted, neither the name nor the value is submitted to the server

see [this MDN

* When it's checked, a query parameter `vip=on` is added to the query string and the dotted pair `(vip . #f)` should be added to the sexp serialization of the post
* when it's unchecked, the query parameter `vip` is absent from the query string and the dotted pair `(vip . #t)` should be added to the sexp serialization of the post