But, but, but, in creating streaming programs it is vital to design
the XPath expressions such that the items being selected are in
document order (no sorting required under the hood). Right?

wrong. The sentence that you quoted in 19.1 is saying exactly that: that
a processor must take care of you and allow you to write //section/head
even though the naive evaluation of that would do the wrong thing, it is
not allowed to be naive.

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