In test accumulator-063 et seq I attempted to create some tests for the rules regarding copying of accumulator values in §18.2.10. For example -063 has the code:

  <xsl:template match="b">
    <xsl:variable name="x">
      <xsl:copy-of select="." copy-accumulators="yes"/>
    </xsl:variable>
    <xsl:for-each select="$x/*">
      <b-acc>{accumulator-before('f:elementNr')}</b-acc>
    </xsl:for-each>
  </xsl:template>

Looking at this again, I don't think this has the intended effect. xsl:copy creates a copy of the b element with copied accumulator values, but this element is then attached to a document node constructed for the variable $x, and this (under rule 11 of Constructing Complex Content) does not copy the accumulator values to the new documnt node.

Although this might seem surprising, the alternative would be worse: consider

    <xsl:variable name="x">
      <a/>
      <xsl:copy-of select="." copy-accumulators="yes"/>
      <b/>
    </xsl:variable>

and ask what the accumulator value should be for element b.

I will rewrite the tests accordingly.

Since I misunderstood the effect of the spec here, others might also do so, and a word of explanation in an erratum might not go amiss.