struct IsPresent(bool);

impl FromFormValue<'v> for IsPresent {
    type Error = ();
    fn from_form_value(form_value: &'v RawStr) -> Result<Self, Self::Error> {
        // from_form_value is only called when a key is present

        Ok(IsPresent(true))
    }

    fn default() -> Option<Self> {
        // default is called after all parameters have been looked at
        // and the given key was found missing. Without default(),
        // the missing key would be treated as an error.
        Some(IsPresent(false))
    }
}