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The text and Note say (partially): <quote> Writing the expression as for $v in S return R, the two operand roles are S and R. <snip /> <note> The rule that R must be grounded prevents the variable being bound to a node in a streamed document. This disallows expressions of the form for $x in child::section return $x/para, because this requires data flow analysis (tracing from the binding of a variable to its usages), rather than purely syntactic analysis. Some implementations may be able to stream such constructs. </note> </quote> But this is not (entirely) true. It is "S" that must be grounded that prevents the variable from being bound to a node. The rest of the text seems correct. Note that we could make "for $n in para return copy-of($n/@indent)" streamable, because the operand R is motionless, but currently we disallow this.
The correction was accepted and has been applied.