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In discussing compatibility of overriding functions, ยง3.5.3.3 says If the overridden function specifies streamable="yes" then the overriding function also specifies streamable="yes", and in addition, it has the same posture and sweep as the function that it overrides. What it means to say is: The explicit or implicit value of the streamability attribute in the overriding function must be the same as the explicit or implicit value of the streamability attribute in the overridden function.
I actually think this was deliberate, I remember (but cannot readily find the reference) that we discussed that a non-streamable stylesheet function can be overridden by a streamable stylesheet function, but not the other way around. Hence the current rule is written specifically to refer only to the situation where streamable="yes" is specified. Your change would suggest that that is no longer the case and would make this change more than just editorial.
OK, but my main point was that xsl:function does not have an @streamable attribute, it has an @streamability attribute. Also, a function per se does not have a posture and sweep. Perhaps the correct text is: If the overridden function has a streamability attribute with a value other than "unspecified", then the overriding function must have a streamability attribute with the same value.
I didn't realize that, yes that makes sense.
Covered by draft erratum E32, see https://htmlpreview.github.io/?https://github.com/w3c/qtspecs/blob/master/errata/xslt-30/html/xslt-30-errata.html#E32