let x be set ; :: according to YELLOW_1:def 3 :: thesis: ( not x in proj2 (MSSet (P,Q)) or x is RelStr )
assume x in rng (MSSet (P,Q)) ; :: thesis: x is RelStr
then x in {P,Q} by FUNCT_4:64;
hence x is RelStr by TARSKI:def 2; :: thesis: verum