reserve k,n,i,j for Nat;

theorem Th18:
  for p being Element of Permutations(n) holds p" is Element of Permutations(n)
proof
  let p be Element of Permutations(n);
  p" is Element of Group_of_Perm(n) by MATRIX_1:14;
  hence thesis by MATRIX_1:def 13;
end;
