theorem Th18:
  for s,s9 being convergent Complex_Sequence
  holds lim(s - s9)=(lim s)-( lim s9)
proof
  let s,s9 be convergent Complex_Sequence;
  lim(s - s9) = (lim s) + lim(-s9) by Th12
    .= (lim s) + -(lim s9) by Th16;
  hence thesis;
