SSI and XHTML

>Hi! I'm not an expert in XSL and XHTML so I need a little help!
>
>I have an XSL file that ends up using the ENTITY (varname) SYSTEM
>(file.xhtml) and then calls this variable which in turn outputs the
>content in this .xhtml.
>
>The problem is that in the XHTML file I want to do include. I tried
>doing a Server Side Include (SSI), but XHTML seems to ignore it. So
>how can I do an include on a file from inside an XHTML file?