reserve Y for non empty set;

theorem
  for a,b being Function of Y,BOOLEAN holds ('not' a 'or' 'not' b)
  'imp' 'not'( a '&' b)=I_el(Y)
proof
  let a,b be Function of Y,BOOLEAN;
  thus ('not' a 'or' 'not' b) 'imp' 'not'( a '&' b) = 'not'( a '&' b) 'imp'
  'not' (a '&' b) by BVFUNC_1:14
    .= 'not'( a '&' b) 'imp' ('not' a 'or' 'not' b) by BVFUNC_1:14
    .=I_el Y by Th36;
end;
