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

[Robert Felps]

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.