theorem Th33:
   x = Class(EqRel(S),a) & y = Class(EqRel(S),b) implies
     x*y = Class(EqRel(S),a*b)
   proof
     assume that
A1:  x = Class(EqRel(S),a) and
A2:  y = Class(EqRel(S),b);
     consider a1, b1 being Element of Frac(S) such that
A3:  x = Class(EqRel(S),a1) and
A4:  y = Class(EqRel(S),b1) and
A5:  (the multF of S~R).(x,y) = Class(EqRel(S),a1*b1) by Def6;
A6:  a1,a Fr_Eq S by A1,A3,Th26;
     b1,b Fr_Eq S by A2,A4,Th26;
     hence thesis by A5,Th26,A6,Th27;
   end;
