<xsl:for-each select="marc:datafield[@tag='035']">
<pz:metadata type="system-control-nr">
- <xsl:value-of select="marc:subfield[@code='a']"/>
+ <xsl:choose>
+ <xsl:when test="marc:subfield[@code='a']">
+ <xsl:value-of select="marc:subfield[@code='a']"/>
+ </xsl:when>
+ <xsl:otherwise>
+ <xsl:value-of select="marc:subfield[@code='b']"/>
+ </xsl:otherwise>
+ </xsl:choose>
</pz:metadata>
</xsl:for-each>