xsl:for-each...iteration question

This has been answered before I'm sure but doing a relevant search in the
archives is tricky.

I've an input doc containing a number of modify-attr elements
<modify>
  <modify-attr attr-name = "abc"/>
  <modify-attr attr-name = "123"/>
  <modify-attr attr-name = "789"/>
</modify>

I have another document containing a list of prohibited attributes.
<exclude-list>
  <exclude-attr>123</exclude-attr>
</exclude-list>


I want to iterate over my input doc and copy all those modify-attr elements
that are NOT in the exclude list to the output.

<xsl:template match="modify/modify-attr">
  <xsl:variable name="currentAttr" select="@attr-name"/>
  <xsl:variable name="currentNode" select="."/>

  <xsl:for-each select="document('excludedoc')/exclude-list/exclude-attr">
    <xsl:when test="$currentAttr=.">
      <xsl:message>Exclude</xsl:message>
    </xsl:when>
    <xsl:otherwise>
      <xsl:copy-of select="$currentNode"/>
    </xsl:otherwise>
  </xsl:for-each>

</xsl:template>

Obvisouly, this is flawed, I get multiple modify-attr elements copied to the
output because I cannot see a way of ensuring the for-each only copies a
permitted node once and once only.

I've a feeling this is approaching the problem from the wrong way....