public void
present(Record record) {
	newPrereq(record);
}
Inserts a prerequisite that the specified record exists.  Due to the way
value-dependent prequisites work, the condition that must be met is that
the set of all records with the same and type in the update message must
be identical to the set of all records with that name and type on the server.
src/main/java/core/org/xbill/DNS/Update.java