xslt node-set use in key function - find children by parent attribute

I have an XML File.

<?xml version="1.0" encoding="UTF-8"?>
<XML>   
    <Items>
        <ItemRef ID='answer'></ItemRef>
        <ItemRef ID='number'></ItemRef>
        <ItemRef ID='delete'></ItemRef>
        <ItemRef ID='remove'></ItemRef>
    </Items>
    <ItemDef ID='anwser'></ItemDef>
    <ItemDef ID='number'>
        <EnumItemRef EID = '1'></EnumItemRef>
    </ItemDef>
    <ItemDef ID='delete'>
        <EnumItemRef EID = '2'></EnumItemRef>
    </ItemDef>        
    <ItemDef ID='remove'></ItemDef>
    <EnumItem EID="1">
        <EnumItem Value = '1'></EnumItem>
        <EnumItem Value = '2'></EnumItem>
        <EnumItem Value = '3'></EnumItem>
    </EnumItem>
    <EnumItem EID="2">
        <EnumItem Value = 'yes'></EnumItem>
        <EnumItem Value = 'no'></EnumItem>
    </EnumItem>
</XML>

what I need to transform into a XML file like this:

<?xml version="1.0" encoding="UTF-8"?>
<XML>   
    <Items>
        <ItemRef ID='newid_answer'></ItemRef>
        <ItemRef ID='newid_number'></ItemRef>
    </Items>
    <ItemDef ID='newid_answer'></ItemDef>
    <ItemDef ID='newid_number'>
        <EnumItemRef EID = '1'></EnumItemRef>
    </ItemDef>        
    <EnumItem EID="1">
        <EnumItem Value = '1'></EnumItem>
        <EnumItem Value = '2'></EnumItem>
        <EnumItem Value = '3'></EnumItem>
    </EnumItem>
</XML>

I have following XSLT 1.0 file. With this I'm able to create the Items and ItemDefs I want. I'm just missing an idea how to find the corresonding EnumItemRef and EnumItem. Either I have to check if the current ItemDef has a child called EnumItemRef, or I could solve it with a key function (what would maybe be the better solution). I tried both but I can't make it work.

<xsl:variable name="Items">
    <Item ID="answer">newid_answer</Item>
    <Item ID="number">newid_number</Item>
</xsl:variable>

<xsl:key name="get_Enum_by_ID" match="/XML/ItemDef/EnumItemRef" use="/XML/ItemDef/@ID" />

<xsl:template match="XML">
    <xsl:copy>
        <xsl:apply-templates />
        <xsl:call-template name="Items" />
        <xsl:call-template name="ItemDef" />
    </xsl:copy>
</xsl:template>

<xsl:template name="Items">
        <xsl:element name="Items">
        <xsl:for-each select="exslt:node-set($Items)/Item">
            <xsl:element name="ItemRef">
                <xsl:attribute name="ID"><xsl:value-of select="text()"/></xsl:attribute>
            </xsl:element>
        </xsl:for-each>
    </xsl:element>
</xsl:template>

<xsl:template name="ItemDef">
    <xsl:for-each select="exslt:node-set($Items)/Item">
        <xsl:element name="ItemDef">
            <xsl:attribute name="ID"><xsl:value-of select="text()" /></xsl:attribute>
        </xsl:element>
        <-- how can I find the corresponding EnumitemRef and EnumItem? -->
        <xsl:copy-of select="key('get_Enum_by_ID', @ID)" /> 

    </xsl:for-each>
</xsl:template>

I would be happy if anyone could give me an idea how to solve this.