why can't you exportfs /dir and /dir/sub?

[Alex Miroshnichenko]

In article <felps.676828858 at convex.convex.com>, felps at convex.com (Robert
Felps) writes:
|> 
|> Why can't this be done?
|> 
|> EXPORTFS(8)        NFS Programmer's Reference        EXPORTFS(8)
|> 
|> WARNINGS
|>      You cannot export a directory that is either a parent- or a
|>      sub-directory of one that is currently exported and within
|>      the same filesystem. It would be illegal, for example, to
|>      export both /usr and /usr/local if both directories resided
|>      in the same disk partition.

exportfs is performed on a filesystem, not on a directory tree.