How should we read the '?' in this case?
   Convert to string if you can, else don't bother?

no it should be read the way ? is read in regex or dtd syntax as 0-or-1
you could also use + or * there, again with their regex or dtd meanings
of 0-or-more or 1-or-more.

a type of xs:string requires a value that is a string.
a type of xs:string? requires a value that is a sequence of 0 or more
strings. (as always though, there is no difference between a single
string and a sequence of length 1 that contains a string)


David

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