Fix: Handle RNC for alts elements in GMP doc

When generating the RNC schema for elements, handle the case where
an alts type is used.

This fixes the schema showing "ERROR" instead of the expected pattern.

src/schema_formats/rnc.xsl

@@ -305,6 +305,25 @@ response
           <xsl:with-param name="parent-name" select="$parent-name"/>
         </xsl:call-template>
       </xsl:when>
+      <xsl:when test="name()='alts'">
+        <xsl:for-each select="alt">
+          <xsl:choose>
+            <xsl:when test="following-sibling::alt and preceding-sibling::alt">
+              <xsl:text>|</xsl:text>
+              <xsl:value-of select="."/>
+            </xsl:when>
+            <xsl:when test="count (following-sibling::alt) = 0">
+              <xsl:text>|</xsl:text>
+              <xsl:value-of select="."/>
+              <xsl:text>" }</xsl:text>
+            </xsl:when>
+            <xsl:otherwise>
+              <xsl:text>xsd:token { pattern = "</xsl:text>
+              <xsl:value-of select="."/>
+            </xsl:otherwise>
+          </xsl:choose>
+        </xsl:for-each>
+      </xsl:when>
       <xsl:when test="name()='t'">
       </xsl:when>
       <xsl:otherwise>