theorem Th130:
  |(p1 + p2, p3)| = |(p1, p3)| + |(p2, p3)|
proof
  reconsider f1=p1, f2=p2, f3=p3 as real-valued FinSequence;
  len f2=n by CARD_1:def 7;
  then len f1=len f2 & len f2=len f3 by CARD_1:def 7;
  hence thesis by Th120;
end;
