fxnilgnilxfxgyzgfxyz++xnilx++xgyzg++xyznullniltruenullgxyfalsememnilyfalsememgxyzor=yzmemxzmemxmaxxnotnullxmaxggnilxymax'xymaxgggxyzumax'maxggxyzufxgyzfxy++xgyz++xymemgxyzmemxzmemxmaxxnullxmaxgggxyzumaxggxyztruefxgyzfxytrue31f212g2111f2112nil01++21112null1111true01false01mem211112or21=20max1111not111max'21u00++xgyz++xytrue1++212g2111f2112nil01++21112null1111true01false01mem211112or21=20max1111not111max'21u00memgxyzmemxztrue1mem211g2111f2112nil01++21112null1111true01false01mem211112or21=20max1111not111max'21u00memxmaxxnullxfalsemaxgggxyzumaxggxyztrue1max111g2111u00f2112nil00++21112null10true00false00mem21or20=20max11not10max'20