Question about all, choice, and sequence

Hello,

I have a question about the "all"-, "choice"-, and "sequence"-sequences.
What are the constraints on Schema definitions of these model groups?
A choice which consists only of one element isn't really a choice but
rather one required element, so is a choice with only one element allowed?
Where do I find the constraints on model groups in the Schema spec? The
section 3.7 "Model Group Definitions" didn't make it clear to me.

thanks
   Heiko 

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