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The type-determined usage for union types should be the same as for atomic types, because they trigger atomization in the same way as atomic types. (It is currently the same as for nodes).
Related, there are two small typos, both under 19.8.9.8 (fn:fold-left) and 19.8.9.10 (fn:for-each) where "type determined usage" is written (without hyphen) and not linked to the definition. I agree to the suggestion in the bug description. Note that under 6.6.4 Streamable Templates, this was already recognized (but see bug 28154, this should become type-adjusted posture and sweep): <quote> The xsl:template element has an as attribute that specifies an atomic or union type. </quote>
The change was accepted and has been applied.