Add attribute to copied element

How can I copy elements from a key and add an attribute to every copied
element containing its position in the sequence.

E.g. The key "contains" 3 my-include tags:
<my-include src="a"/>
<my-include src="b"/>
<my-include src="c"/>

As output I want
<my-include src="a" position="1"/>
<my-include src="b" position="2"/>
<my-include src="c" position="3"/>

Right now I have:
<xsl:copy-of select="key('textItems',generate-id())[name() =
'my-include']"/>
<xsl:attribute name="position">
	<xsl:value-of select="position()"/>
</xsl:attribute>

But I know its wrong. How could I do this?

Robert