E1-5 missed a case

E1-5 cleanup up the interaction between empty and mixed and so on for,
among other things, derivation by extension.

There is one case we still arguably don't allow that we should -- you
can't extent empty to mixed.  It doesn't violate the prefix invariant,
so seems to be it should be allowed.

Suggested candidate requirement for 1.1.

ht
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  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
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Received on Thursday, 4 December 2003 07:49:38 UTC