( dom (R | X) c= dom R & dom R c= A ) by Def18, Th89;
hence dom (R | X) c= A by XBOOLE_1:1; :: according to RELAT_1:def 18 :: thesis: R | X is X -defined
thus dom (R | X) c= X by Th87; :: according to RELAT_1:def 18 :: thesis: verum