theorem Th53:
  f is_right_convergent_in x0 implies -f is_right_convergent_in x0
  & lim_right(-f,x0)=-(lim_right(f,x0))
proof
  assume
A1: f is_right_convergent_in x0;
  thus -f is_right_convergent_in x0 by A1,Th52;
  thus lim_right(-f,x0)=(-1)*(lim_right(f,x0)) by A1,Th52
    .=-(lim_right(f,x0));
end;
