reserve p,q,r for FinSequence;
reserve u,v,x,y,y1,y2,z for object, A,D,X,Y for set;
reserve i,j,k,l,m,n for Nat;

theorem
  idseq n | Seg m = idseq n iff n <= m
proof
  len idseq n = n by CARD_1:def 7;
  hence thesis by Th47;
end;
