RE: Possible to pass 'empty sequence' to function?

M.K:
> It's presumably failing because you declared the type on
> xsl:param not to accept an empty sequence. For example if you
> write <xsl:param name="p" as="xs:string"/> then an empty
> sequence will not be accepted. If you change it to <xsl:param
> name="p" as="xs:string?"/> then an empty sequence is OK.

When is it a good idea to type a variable/param as "xs:string?" - what
benefits does it bring over not specifying the "as" attribute?

thanks
andrew