(newbie) checking for which buffers are visible to the user

[Stuart McLean]

Hi,

I have bound f6 to bring up the calendar. Here is what I would like to
do. If the calendar is not visible to me, make it visible. However, if
I can see it, I would like to bury the calendar buffer. I am not sure
the most effective way to check if a buffer is visible to the user
(me) and how to write a function that takes advantage of this.

Does anyone have any pointers? I have spent several hours checking the
documentation of several window and buffer related functions, but none
of them (I think) seems to do what I want. Any help would be very
welcome. 

Thanks,

Stuart