Q: How can I swap buffers in two windows?

[Eric Pement]

This request has appeared several times on this newsgroup, worded in
different ways, but has never been answered. I have already checked the
FAQ, M-x apropos, and the Emacs info pages. Can someone please give
help or direction here?

I have an Emacs frame split into two windows, which may be split
vertically or horizontally. I would like a single command to swap or
exchange both windows, i.e., put buffer A into window B, and put buffer
B into window A. If there are not exactly two windows in the frame,
issue an error message.

I know that I can do "C-x b", {B}, <RET>, "C-x o", "C-x b", {A}, <RET>,
but I want to execute it with a single command without typing out the
buffer names at all. Emacs already knows the names of the buffers in
each window, so why should I have to type them?

My elisp skills are not well-developed, so I'm asking for help. I have
GNU Emacs version 21.3.50.8. Thanks in advance.

--
Eric Pement