theorem
  |.r*seq.| = |.r.|(#)|.seq.|
proof
  let n be Element of NAT;
  thus |.r*seq.|.n=|.(r*seq).n.| by Def6
    .=|.r*(seq.n).| by Th5
    .=|.r.|*|.seq.n.| by Th7
    .=|.r.|*(|.seq.|).n by Def6
    .=(|.r.|(#)|.seq.|).n by SEQ_1:9;
end;
