This element represents a requirement for an issued token, that is one issued by some token
issuer using the mechanisms defined in WS-Trust.
author: K.Venugopal@sun.com
returns a
java.util.Iterator over the token reference types to be used.
either REQUIRE_KEY_IDENTIFIER_REFERENCE,REQUIRE_ISSUER_SERIAL_REFERENCE,REQUIRE_EMBEDDED_TOKEN_REFERENCE,REQUIRE_THUMBPRINT_REFERENCE