/usr/share/xsessions/IceWM.desktop is in icewm 1.3.7-1ubuntu1.
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]
Encoding=UTF-8
# The names/descriptions should really be better
Name=IceWM
Comment=This is the window manager IceWM
Comment[de]=Dies ist der Fenstermanager IceWM
Exec=/usr/bin/icewm-session
Icon=icewm.xpm
Type=Application
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