xml entities

sorry if this is off topic... I am trying to get external entities to be
styled by  xsl but I am running into a basic problem.

I am wondering if someone can tell me how to do something like:
<!DOCTYPE apage[
    <!ENTITY logoid "0001">
    <!ENTITY logo SYSTEM "sections/logo/l_xx_&logoid;.xml">
]>

where the first entity defines a "variable" for the next entity.  This does
not work as is, but if I take out SYSTEM it will give me the correct string.
IS there a way to put variable information into a SYSTEM entity?


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