/usr/share/xsessions/flwm.desktop is in flwm 1.02+cvs20080422-10.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 | [Desktop Entry]
Encoding=UTF-8
Type=XSession
Exec=flwm
TryExec=flwm
Name=flwm
Comment=The Fast Light Window Manager
|