Sorry, can't be done in schema (well maybe in certain situations you can kludge it with xsd:keyref or xsd:unique). It is a well-known request that has been defered to a later version of schema.
Generic Schema questions are best asked on [EMAIL PROTECTED] (see the mailing lists link from http://www.w3.org). This mailing list has been deprecated in favor of xerces-j-user and xerces-c-dev, use those mailing lists for issues that are specific to the particular Xerces parser.
