The limits of move operation in shared-subtree

Hi, all: I am reading the implementation of shared-subtree in linux kernel. There are two limits about move operation confused me badly, I have read the design document -- Shared-Subtree Concept, Implementation, andApplications in Linux ,but still puzzled. 1.Why can't move a mount residing in a shared mounts?2.Why can't move a mount tree containing an unbindable mount to a shared mount?Is it not a good way to prune out unbindable mounts just like rbind semantics do? Thanks in advanceRock Lee--------------------------------