reserve x, y for object, X for set,
  i, j, k, l, n, m for Nat,
  D for non empty set,
  K for commutative Ring,
  a,b for Element of K,
  perm, p, q for Element of Permutations(n),
  Perm,P for Permutation of Seg n,
  F for Function of Seg n,Seg n,
  perm2, p2, q2, pq2 for Element of Permutations(n+2),
  Perm2 for Permutation of Seg (n+2);

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;
