Add rend="URN" attribute to idno elements of type URN

see #24
diff --git a/test/test-xml.sh b/test/test-xml.sh
index f0a84c9..32333d7 100755
--- a/test/test-xml.sh
+++ b/test/test-xml.sh
@@ -44,6 +44,12 @@
 observed=$(xmlstarlet sel --net -t -v "count(/idsCorpus/idsDoc/idsText/idsHeader/fileDesc/publicationStmt/idno)"  target/dnb18.i5.xml)
 assert_gt "$observed" "$min_expected" "exvery text has more than one idno element"
 
+observed=$(xmlstarlet sel --net -t -v "count(/idsCorpus/idsDoc/idsText/idsHeader/fileDesc/publicationStmt/idno[@type='URN'])"  target/dnb18.i5.xml)
+assert_eq "$observed" "$min_expected" "exvery text has one idno element of type URN"
+
+observed=$(xmlstarlet sel --net -t -v "count(/idsCorpus/idsDoc/idsText/idsHeader/fileDesc/publicationStmt/idno[@type='URN' and @rend='URN'])"  target/dnb18.i5.xml)
+assert_eq "$observed" "$min_expected" "every idno element of type URN also has attribute rend='URN'"
+
 exit_with_test_summary
 
 
diff --git a/xslt/epub2i5.xsl b/xslt/epub2i5.xsl
index 2c59831..734e644 100644
--- a/xslt/epub2i5.xsl
+++ b/xslt/epub2i5.xsl
@@ -333,6 +333,9 @@
                                             <idno type="{$type}"><xsl:value-of select="replace(., '(([0-9]|-)+X?).*', '$1')"/></idno>
                                         </xsl:if>
                                     </xsl:when>
+                                    <xsl:when test="$type='URN'">
+                                        <idno rend="URN" type="URN"><xsl:value-of select="."/></idno>
+                                    </xsl:when>
                                     <xsl:otherwise><idno type="{$type}"><xsl:value-of select="."/></idno></xsl:otherwise>
                                 </xsl:choose>
                             </xsl:for-each>