On having recovered a book from a server xserver, this sends to us the ISBN in the code 020 , but on having examined the register of the citation there appears the field empty 'ISBN'.
Examining the file xserver2sakaibrary.xsl we see that 022 identifies the code both with ISSN and with ISBN, when it corresponds to this one 020.
The soluction has been:
<!-- GET ISSN/ISBN -->
select="datafield[(@tag='022' or @tag='020') and @ind1=' ' and @ind2=' ']/subfield[@code='a']">
<xsl:value-of select="text()" />