/usr/include/x86_64-linux-gnu/ is in nx-x11proto-xext-dev 2:3.5.99.16-1.
This file is owned by root:root, with mode 0o755.
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/usr/include/x86_64-linux-gnu/nx-X11/
/usr/include/x86_64-linux-gnu/nx-X11/extensions/
/usr/include/x86_64-linux-gnu/nx-X11/extensions/Xdbeproto.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/bigreqstr.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/dpms.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/dpmsstr.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/record.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/recordstr.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/security.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/securstr.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/shapeconst.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/sync.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/syncstr.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/xcmiscstr.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/xf86bigfont.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/xf86bigfproto.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/xtestconst.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/xtestext1.h
/usr/include/x86_64-linux-gnu/nx-X11/extensions/xteststr.h