theorem Th25:
  x in Class(EqRel(S),y) iff x,y Fr_Eq S
  proof
    set E = EqRel(S);
    hereby
      assume x in Class(E,y);
      then [x,y] in E by EQREL_1:19;
      hence x,y Fr_Eq S by Def5;
    end;
    assume x,y Fr_Eq S;
    then [x,y] in E by Def5;
    hence thesis by EQREL_1:19;
  end;
