theorem
  y<>0.SF & z<>0.SF implies x/(y/z)=(x*z)/y
proof
  assume
A1: y<>0.SF;
  assume
A2: z<>0.SF;
  then z"<>0.SF by Th13;
  hence x/(y/z)=x*(z""*y") by A1,Th11
    .=x*(z*y") by A2,Th14
    .=(x*z)/y by GROUP_1:def 3;
end;
