[]
mount Filesystem(1)
[(Filesystem(1), '/foo')]
mount Filesystem(2)
[(Filesystem(1), '/foo'), (Filesystem(2), '/bar/baz')]
mount Filesystem(3)
[(Filesystem(1), '/foo'), (Filesystem(2), '/bar/baz'), (Filesystem(3), '/bar')]
umount Filesystem(2)
[(Filesystem(1), '/foo'), (Filesystem(3), '/bar')]
mount Filesystem(4)
[(Filesystem(1), '/foo'), (Filesystem(3), '/bar'), (Filesystem(4), '/')]