/usr/share/xsessions/gnome-flashback-xmonad.desktop is in xmonad 0.12-1.
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 8 9 | [Desktop Entry]
# Maintained by Per Olofsson <pelle@debian.org>
Name=GNOME Flashback (Xmonad)
Comment=This session logs you into GNOME Flashback with Xmonad
Exec=gnome-flashback-xmonad
TryExec=gnome-flashback
Icon=
Type=Application
DesktopNames=GNOME-Flashback;GNOME;
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