[xsl] How to copy only the really existing <choice> branch/element? "val

Assume I have a XML source based on the following (simplified) XSD schema:

<xsd:element name="top" type="mytype"/>

<xsd:complexType name="mytype">
  <xsd:choice>
    <xsd:element name="aaa" type=".....">
    <xsd:element name="bbb" type=".....">
    <xsd:element name="ccc" type=".....">
  </xsd:choice>
</xsd:complexType>

That mean either element aaa or element bbb or element ccc is filled/exists.

Now I want to copy only this sub-element to a target which currently exists.

If I simply write:

<xsl:value-of select=".../aaa"/>
<xsl:value-of select=".../bbb"/>
<xsl:value-of select=".../ccc"/>

then ALL three elements were copied. The 2 branches which do not exist were 
created
as empty elements so that the target XML doc looks like e.g.

<top>
<aaa></aaa>
<bbb>myvalue</bbb>
<ccc></ccc>
</top>

which is an invalid XML because of its XSD schema file definition.

The question is now: How can I let XSL copy only this branch/element which 
really exists?
I need a function like

<xsl:value-of-only-the-existing-choice-branch select=..../top"/>

or

<xsl:value-of-if-exists select=".../aaa"/>
<xsl:value-of-if-exists select=".../bbb"/>
<xsl:value-of-if-exists select=".../ccc"/>

How can I do this in detail?

Thank you
Ben



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