[relax-ng] Interleave and text

I have been trying to figure out whether we need to make it an error if
<text/> occurs in both operands of an <interleave/>.  So far I haven't had
much success.  I haven't been able to construct an example that causes Jing
to blows up nor have I been able to convince myself that it is impossible to
construct such an example. Help!

James