reserve n for Element of NAT,
  p,q,r,s for Element of HP-WFF;

theorem Th26:
  for A,B being non empty set for P being Permutation of A,
      Q being Permutation of B holds (P => Q)" = P" => (Q")
proof
  let A,B be non empty set;
  let P be Permutation of A, Q be Permutation of B;
  now
    let f be Element of Funcs(A,B);
    thus (P => Q)".f = Q"*f*P by Th25
      .= Q"*f*P"" by FUNCT_1:43
      .= (P" => (Q")).f by Def1;
  end;
  hence thesis;
end;
