theorem
  f1/f + f2/f = (f1 + f2)/f & f1/f - f2/f = (f1 - f2)/f
proof
  thus f1/f + f2/f = f1(#)(f^) +f2/f by Th31
    .= f1(#)(f^) + f2(#)(f^) by Th31
    .= (f1+f2) (#) (f^) by Th10
    .= (f1+f2)/f by Th31;
  thus f1/f - f2/f = f1(#)(f^) - f2/f by Th31
    .= f1(#)(f^) -f2(#)(f^) by Th31
    .= (f1-f2)(#)(f^) by Th14
    .= (f1-f2)/f by Th31;
end;
