Count Distinct Elements based on Attributes - XSL

[Jane]

Hi,
I have the following XML:

<DailySchedule>
<Appointment AttendedBy="Mr Judith Chalmers" ApptTime="9:15"/>
<Appointment AttendedBy="Mr Johnny Vegas" ApptTime="13:15"/>
<Appointment AttendedBy="Mr Johnny Vegas" ApptTime="15:15"/>
<Appointment AttendedBy="Mrs Albert Einstein" ApptTime="15:15"/>
<Appointment AttendedBy="Mrs Albert Einstein" ApptTime="13:15"/>
<Appointment AttendedBy="Mr Judith Chalmers" ApptTime="15:15"/>
</DailySchedule>

and what I want to do is highlight a TD in a different colour if all
the users have an appointment at one time. For instance, in the above
example, Johnny Vegas, Albert Einstein and Judith Chalmers all have
15:15 appointments, so I would make this TD a different colour in my
table. But I do not want to do it for the time of 13:15 as only two
of the three AttendedBy's have this appointment time.

I think I need some kind of sort/count combination but am not sure
what!

Is is possible to do this please?

thanks
Jane