theorem Th3:
  for n st n >= 2 holds {1,2} in 2Set Seg n
proof
  let n such that
A1: n>=2;
  1<=n by A1,XXREAL_0:2;
  then
A2: 1 in Seg n;
  2 in Seg n by A1;
  hence thesis by A2,Th1;
end;
