theorem
  t<>0 implies t(.)(X (o) Y) = t(.)X (o) t(.)Y
proof
  assume t<>0;
  then t(.)(X (o) Y) =t(.)(X (+) Y) (-) t(.)Y by Th64
    .=t(.)X (+) t(.)Y (-) t(.)Y by Th63;
  hence thesis;
end;
