/usr/lib/mono/xbuild/ is in mono-xbuild 4.2.1.102+dfsg2-7ubuntu4.
This file is owned by root:root, with mode 0o755.
..
/usr/lib/mono/xbuild-frameworks/.NETFramework/
/usr/lib/mono/xbuild-frameworks/.NETFramework/v2.0/
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