reserve Y for non empty set,
  a,b,c,d for Function of Y,BOOLEAN;

theorem Th21:
  a 'imp' ('not' a 'imp' a) = I_el(Y)
proof
  a 'imp' ('not' a 'imp' a) ='not' a 'or' ('not' a 'imp' a) by BVFUNC_4:8
    .='not' a 'or' ('not' 'not' a 'or' a) by BVFUNC_4:8
    .='not' a 'or' a;
  hence thesis by BVFUNC_4:6;
end;
