We say that an xsl:element instruction (say) with a use-attribute-sets attribute can be streamable if the attribute set is streamable.

This assumes that at the time we do the streamability analysis, we know which attribute-set will be used. But since this binding is not determined until all the packages making up the stylesheet have been assembled, the result is that we can't assess streamability on a single package individually. This seems unfortunate, especially as streaming may cause radical changes to the way a package is compiled.

In practice most attribute sets will be motionless so it's a shame to disallow their use simply because of the remote possibility of free-ranging behaviour.

We could add an attribute streamable=yes|no to xsl:attribute-set and say that if streamable=yes is specified, the body must be motionless.