[plt-scheme] Determining if scheme type is boolean

Dear All,

I'm trying to determine if a type of a given Scheme_Object to a
C function is of type 'boolean', but SCHEME_FALSEP/SCHEME_TRUEP
don't do their job good enough. In fact, if I give (seconds->date 42)
to the function, it will yield true on SCHEME_TRUEP and thereby
grant this different type status 'boolean'.

What can I do about this? I really need to distinguish between boolean
and other types in my function (mzgtk2-set-property).

Best regards,

Hans