theorem
  for p,q being FinSequence st q = p|Seg n holds len q <= n
proof
  let p,q be FinSequence;
  assume q = p|Seg n;
then A1: dom q = dom p /\ Seg n by RELAT_1:61;
 Seg len q = dom q by Def3;
  hence thesis by Th5,A1,XBOOLE_1:17;
end;
