Perhaps the most important rule on streamability is found in 19.10 and says that:


1) If a construct is guaranteed-streamable then it MUST be processed using streaming.

This text, and the text around it, do not seem to say that when the supplied node is itself not a streamed node, then it will not be processed using streaming (consider the input to IMS, or a copy-of() call, or an apply-templates with a non-streamed input tree to a streamable mode).

I propose something along the following:

1) If a construct is guaranteed-streamable and the input tree is provided as a streamed node, then it MUST be processed using streaming.

I think this takes care of the scenarios where an input tree is not a streamed tree, or where a calling construct creates a non-streamed tree, or is itself invoked with a non-streamed tree.

We could add a Note that says that it is an API matter how a processor provides a streamed node as input. Note also that we do not forbid a non-streamed node as input (except for xsl:stream and xsl:merge-source).