Question about the meaning of tai6464

Hi,

I'm interested in implementing the io library interface
(http://cr.yp.to/lib/io.html), but have a few questions.   First of
all, many of the io functions use "tai6464" datatypes.  Looking at the
tai interface, as well as the taia interface, I don't see tai6464
mentioned anywhere.  I also don't see a single instance of the word
'"tai6464" in *any* code available on the entire cr.yp.to website.  I
have seen a few other projects that assume tai6464 is a struct for
TAI64NA data types, but is there an official definition?  This
datatype is also referred to in the npthread interface, where it goes
on to mention (but not define) a few tai6464 functions, such as
tai6464_now().  Is there a tai6464 interface specification?  If so,
where is it?  If not, how does one implement the io library or the
npthread library?

Thanks,
Jay Sullivan