theorem
  seq = - ( - seq )
proof
  now
    let n be Element of NAT;
    thus (- ( - seq )).n = - (- seq).n by BHSP_1:44
      .= - ( - seq.n) by BHSP_1:44
      .= seq.n by RLVECT_1:17;
  end;
  hence thesis by FUNCT_2:63;
end;
