let x be TolStr ; :: according to PCS_0:def 18 :: thesis: ( x in rng (MSSet (P,Q)) implies x is pcs-tol-symmetric )
assume x in rng (MSSet (P,Q)) ; :: thesis: x is pcs-tol-symmetric
then x in {P,Q} by FUNCT_4:64;
hence x is pcs-tol-symmetric by TARSKI:def 2; :: thesis: verum