DTD OR equivalent in XML schema

Hi,

I have been trying to convert some DTD's we are using to the equivalent XML
schema, but I cannot find a way to reflect the DTD OR which states that one
or the other element only can be specified.

i.e. if a document could contain an element A or an element B but not both
the DTD would state something like:

<!ELEMENT document((A|B))>

Can anyone help by suggesting how I can show this in an XML schema?  Please
forgive me if there is an obvious answer to this but I'm new to using
schemas and cant see a simple solution.

Cheers,
Neil.