theorem Th90:
  f is convergent_in-infty implies -f is convergent_in-infty &
  lim_in-infty(-f)=-(lim_in-infty f)
proof
  assume
A1: f is convergent_in-infty;
  thus -f is convergent_in-infty by A1,Th89;
  thus lim_in-infty (-f)=(-jj)*(lim_in-infty f) by A1,Th89
    .=-(lim_in-infty f);
end;
