Fix recursive xsl:call-template producing NaN via result-tree fragment variables

tests/xslt/call-template.test.ts

@@ -0,0 +1,49 @@
+import { Xslt, XmlParser } from '../../src/index';
+
+describe('xsl:call-template', () => {
+    let xslt: Xslt;
+    let xmlParser: XmlParser;
+
+    beforeEach(() => {
+        xslt = new Xslt();
+        xmlParser = new XmlParser();
+    });
+
+    // https://github.com/DesignLiquido/xslt-processor/issues/217
+    it('supports recursion, using a variable to hold the result of a nested call-template', async () => {
+        const xml = xmlParser.xmlParse('<FOO></FOO>');
+        const stylesheet = xmlParser.xmlParse(`<?xml version="1.0" encoding="utf-8"?>
+<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+    <xsl:output method="xml" indent="yes"/>
+
+    <xsl:template name="factorial">
+        <xsl:param name="number"/>
+        <xsl:choose>
+            <xsl:when test="$number = 1">
+                <xsl:value-of select="1"/>
+            </xsl:when>
+            <xsl:otherwise>
+                <xsl:variable name="temp">
+                    <xsl:call-template name="factorial">
+                        <xsl:with-param name="number" select="$number - 1"/>
+                    </xsl:call-template>
+                </xsl:variable>
+                <xsl:value-of select="$number * $temp"/>
+            </xsl:otherwise>
+        </xsl:choose>
+    </xsl:template>
+
+    <xsl:template match="FOO">
+        <QUX>
+            <xsl:attribute name="factorial">
+                <xsl:call-template name="factorial">
+                    <xsl:with-param name="number" select="5"/>
+                </xsl:call-template>
+            </xsl:attribute>
+        </QUX>
+    </xsl:template>
+</xsl:stylesheet>`);
+        const result = await xslt.xsltProcess(xml, stylesheet);
+        expect(result).toBe('<QUX factorial="120"/>');
+    });
+});