theorem Th13:
  (r*q)*seq = r*(q*seq)
proof
  let n be Element of NAT;
  thus ((r*q)*seq).n=(r*q)*seq.n by Th5
    .=r*(q*seq.n) by RLVECT_1:def 7
    .=r*(q*seq).n by Th5
    .=(r*(q*seq)).n by Th5;
end;
