reserve i,j,k,n for Nat;
reserve x,x1,x2,x3,y1,y2,y3 for set;

theorem Th21:
  for n be Nat, i being Nat st i in the carrier of Necklace n holds i < n
proof
  let n be Nat, i being Nat;
  assume i in the carrier of Necklace n;
  then i in Segm n by Th19;
  hence thesis by NAT_1:44;
end;
