reserve Y for non empty set;

theorem
  for a,b being Function of Y,BOOLEAN holds (a 'imp' 'not' b)
  'imp' (b 'imp' 'not' a)=I_el(Y)
proof
  let a,b be Function of Y,BOOLEAN;
    let x be Element of Y;
A1: now
      per cases by XBOOLEAN:def 3;
      case
        a.x=TRUE;
        hence ('not' a.x 'or' a.x)=TRUE by BINARITH:10;
      end;
      case
        a.x=FALSE;
        then 'not' a.x 'or' a.x =TRUE 'or' FALSE by MARGREL1:11
          .=TRUE by BINARITH:10;
        hence ('not' a.x 'or' a.x)=TRUE;
      end;
    end;
A2: now
      per cases by XBOOLEAN:def 3;
      case
        b.x=TRUE;
        hence ('not' b.x 'or' b.x)=TRUE by BINARITH:10;
      end;
      case
        b.x=FALSE;
        then 'not' b.x 'or' b.x =TRUE 'or' FALSE by MARGREL1:11
          .=TRUE by BINARITH:10;
        hence ('not' b.x 'or' b.x)=TRUE;
      end;
    end;
    ((a 'imp' 'not' b) 'imp' (b 'imp' 'not' a)).x ='not' (a 'imp' 'not' b
    ).x 'or' (b 'imp' 'not' a).x by BVFUNC_1:def 8
      .='not' ('not' a.x 'or' ('not' b).x) 'or' (b 'imp' 'not' a).x by
BVFUNC_1:def 8
      .=('not' 'not' a.x '&' 'not' ('not' b).x) 'or' ('not' b.x 'or' ('not'
    a).x) by BVFUNC_1:def 8
      .=(a.x '&' 'not' 'not' b.x) 'or' ('not' b.x 'or' ('not' a).x) by
MARGREL1:def 19
      .=('not' b.x 'or' 'not' a.x) 'or' (a.x '&' b.x) by MARGREL1:def 19
      .=(('not' b.x 'or' 'not' a.x) 'or' a.x) '&' (('not' b.x 'or' 'not' a.x
    ) 'or' b.x) by XBOOLEAN:9
      .=('not' b.x 'or' TRUE) '&' (('not' b.x 'or' 'not' a.x) 'or' b.x) by A1,
BINARITH:11
      .=TRUE '&' (('not' b.x 'or' 'not' a.x) 'or' b.x) by BINARITH:10
      .=(('not' a.x 'or' 'not' b.x) 'or' b.x) by MARGREL1:14
      .=('not' a.x 'or' TRUE) by A2,BINARITH:11
      .=TRUE by BINARITH:10;
    hence thesis by BVFUNC_1:def 11;
end;
