Proposed resolution for #19: attrInAttr

After studying GNF, I have changed my mind.  Attribute patterns containing 
attribute patterns look strange, but I do not see other technical reasons.

Here is my proposal.  

This syntax allows attribute patterns to  directly or indirectly contain 
other attribute or element patterns, which never match anything.  Implementations 
SHOULD issue warning for such embedded patterns, but this is not an error.

Cheers,

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MURATA Makoto