Help needed in defining a certain constraint in schema.

[Devesh H Rao]

Hi ppl,
I am defining a XML which is to be used for processing rules. The spec for the same contains a function tag having the following structure.

 
                 <Function type="OR" >
<Param id=""/>
<Constant id=""/>
<Handler id="" type="">
                 </Function>

The functionality i am looking for is can i define the tags that are valid inside a function depending on the type of function.
ie. if for example if the type of function is "OR" the function tag can contain X no of child tags and if say the type of function is "isSelected" the function tag can contain only one child tag so on and so forth.
any help will be appreciated.