specifying conditional elements

[aborchers@sdsc.edu]

Is it possible to specify conditonals within an xsd? For example,
consider the following two possible xml inputs:

Input A:
<comparison>
<left_operand>x </left_operand>
<operator>great er_than</operator>
<right_operand> x</right_operand>
</comparison>

Input B:
<comparison>
<left_operand>x </left_operand>
<operator>nul l</operator>
</comparison>


What I would like to do is the following: If the value of <operator> is
either "greater_th an", "less_than" , or "equals", then element
<right_operan d> must be present. If the value of <operator> is "null"
or "not_null" then <right_operan d> should not be present.

My guess is this may be beyond what xsd can handle, but I just thought
I'd ask the experts....

Thanks