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The text under 22.2 Assertions (http://www.w3.org/TR/xslt-30/#assertions), doesn't mention this explicitly, it links to xsl:message for behavior of error-code, see bug 24525. Similarly to that bug, the same applies here in respect to streamability.The streamability analysis of xsl:assert under 19.8.4.6 Streamability of xsl:assert (http://www.w3.org/TR/xslt-30/#streamability-xsl-assert) doesn't mention this AVT, which should be an operand role with usage absorption.
We discussed this in Prague and agreed that these need fixing.