reserve Y for non empty set;
reserve Y for non empty set;

theorem
  for a being Function of Y,BOOLEAN holds a 'imp' 'not' a = 'not' a
proof
  let a be Function of Y,BOOLEAN;
    let x be Element of Y;
    (a 'imp' 'not' a).x ='not' a.x 'or' ('not' a).x by BVFUNC_1:def 8
      .=('not' a).x 'or' ('not' a).x by MARGREL1:def 19
      .=('not' a).x;
    hence thesis;
end;
