> It seems like XSLT should provide a built-in function that evaluates a
sequence (such as the one shown above) for connectedness

isn't that what most of the answers did? take an expression that checks a
pair and then applies it to a sequence using  xpath's every $r in <the
sequence> satisfies <the expression>

It wouldn't be appropriate to have a built in function checking the
specific condition for the specific sequence of non negative integers
possibly separated by -

David