[LC] 5.2.6 xforms:ID-card-number and 7.8.7 The luhn() Function

http://www.w3.org/TR/xforms11/#id-card-type
http://www.w3.org/TR/xforms11/#fn-luhn

"The complementary XPath function luhn() should be used to validate
that the ID number conforms to the specification."

I imagine that almost exclusively these will be used like this:

      <bind nodeset="cc" type="ID-card-number" constraint="luhn()" />

While I applaud the recognition of computer scientists who have
contributed to society, I fear that this is too hard to remember for
this single occurring case. May I propose as easier to cope with
something along the lines of:

      <bind nodeset="cc" type="card-number" constraint="card-number()" />