reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for FinSequence;
reserve D for set;
reserve a, b, c, d, e, f for object;

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;
