[$] An operation for filesystem tucking

As a general rule, the purpose behind mounting a filesystem is to make thatfilesystem's contents visible to the system, or at least to the mountnamespace where that mount occurs. For similar reasons, it is unusual tomount one filesystem on top of another, since that would cause the contentsof the over-mounted filesystem to be hidden. There are exceptions toeverything, though, and that extends to mounted filesystems; a"tucking" mechanism proposed by Christian Brauner is designed to hidemounted filesystems underneath other mounts - temporarily, at least.