reserve Y for non empty set;

theorem
  for a,b being Function of Y,BOOLEAN holds ('not' a '&' 'not' b)
  'imp' 'not'( a 'or' b)=I_el(Y)
proof
  let a,b be Function of Y,BOOLEAN;
  thus ('not' a '&' 'not' b) 'imp' 'not'( a 'or' b) = 'not'( a 'or' b) 'imp'
  'not' (a 'or' b) by BVFUNC_1:13
    .= 'not'( a 'or' b) 'imp' ('not' a '&' 'not' b) by BVFUNC_1:13
    .=I_el Y by Th28;
end;
