<grammar xmlns="http://relaxng.org/ns/structure/1.0"> <start> <element name="addressBook"> <zeroOrMore> <element name="card"> <ref name="cardContent"/> </element> </zeroOrMore> </element> </start> <define name="cardContent"> <element name="name"> <text/> </element> <element name="email"> <text/> </element> </define> </grammar>