let x be RelStr ; :: according to WAYBEL_3:def 8 :: thesis: ( not x in rng (MSSet P,Q) or x is reflexive )
assume x in rng (MSSet P,Q) ; :: thesis: x is reflexive
then x in {P,Q} by FUNCT_4:67;
hence x is reflexive by TARSKI:def 2; :: thesis: verum