conditions in xml schema

Hi,

Is there any way of specifying conditions in XML schema.

Eg.. If element01 is present or element02 is present then element 03 should
be present.
      If either of ele01 or ele02 are present then the other is required

Please clarify

Thanks
Vaidya

PS: I would like to thank those guys who helped me to get xml schema for X12
transaction sets.