Bounds Specification

Okay .... I admit I am not that good at subjects. But I guess you guys
could tell me if there is a way to do this. You know how we can specify
the number of elements using a * or a + as
<!ELEMENT elem (E1*, E2, E3+)>

In the above line, we say that E1 ocuurs 0 or more times, E2 occurs
once and E3 occurs at least once.

I have a situation wherein I need to specify an element E4 that would
be guaranteed to occur 0 or 1 times. 

Someone help me with that ??

Thanks in advance
Sameet

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