Identity constraint on two attribute values

Hi,

Is there a way to say that two attribute values of attributes in an element are not supposed to be the same. My two attributes are both references to the same pool of keys. Those keys are ids of a complex type. Specifically, my element describes an arc, that has references to two points as its "from" and "to" attributes. I don't want the arc to have the same starting and end point.

Thanks,

Kasim