need help demystifying xml error "asynchronous entity"

Hi,

I'm trying to beef up the checks for compare-locales, and I'm running 
into the following:

For an entity like

<!ENTITY foo "stuff with </bad> bar">

I get an "asynchronous entity" error if I use it in a xml doc like

<!DOCTYPE elem [
<!ENTITY foo "stuff with </bad> bar">
]>
<elem>&foo;</elem>

What I'm wondering, is there a way to rephrase that message that at 
least I understand what that means?

I assume that this means that you can't put a start tag in one and the 
end tag in the next entity, but that's just me guessing. I tried to read 
the expat code, and couldn't figure out if I'm guessing right.

Does one of you know?

Thanks

Axel