Resolution for Issue 63

This [1] is a follow-on issue to 62 [2] and I believe this to also 
be resolved in the current draft. The core of this issue is a standard 
failure model for extensions and the mustUnderstand fault code is 
enough to satisfy this requirement.

 --oh

[1] http://www.w3.org/2000/xp/Group/xmlp-issues.html#x63
[2] http://www.w3.org/2000/xp/Group/xmlp-issues.html#x62