How can a client identify the window manager version?

[Xavier Bestel]

Markus Kuhn wrote:
>   - adapting client behaviour to particular window managers (e.g., differently
>     phrased help messages or graphical instructions)

IMHO this is a slippery slope which will introduce dangerous behaviors à
la java: having an app reacting differently depending on the WM causes
subtles and hard-to-debug problems, and is fragile at best (what do you
do for configurable WMs like sawfish, for unknown WMs, for newer
versions with different behavior ?).