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

theorem Th8:
  for X being set, A being Subset of X holds
  ((0,1) --> (1,0))*chi(A`,X) = chi(A,X)
proof
  let X be set, A be Subset of X;
  A`` = A;
  hence thesis by Th7;
end;
