b) Unlike //head, //text() is "striding" - the nodes it selects are
non-overlapping (because text nodes never overlap). So absorption
operations like data(), string(), and <xsl:value-of> are allowed.

It remains to be seen I think how far just having a naive mental model
of the kind of things that ought to work in a streaming context will
lead to expressions that are in fact defined to be streaming in the spec
(or are streaming in the implementations). It's an impressive bit of
analysis/specification in any case, I love the terminology:-)

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