DTD as entity in anther DTD

[hxzhang <hxzhang@cs.ualberta.ca>]

I tried this as found on web:
in one DTD file called "main.dtd", I specified a element
<!ELEMENT main (#PCDATA)> with a lot of subelements

in another DTD file in the same directory, i want to include that element
as an entity like:

<!ENTITY % main SYSTEM "main.dtd">
<!ELEMENT super_main (%main;) >

but this is rejected by parser. Isn't this correct?