theorem Th24:
  -(-f) = f
proof
  thus -(-f) = (-1r)(#)(-f) by Th23
    .= (-1r)(#)((-1r)(#)f) by Th23
    .= ((-1r)*(-1r))(#)f by Th14
    .= f by Th18,COMPLEX1:def 4;
end;
