@article{TM2_2009_XII_2_a1,
     author = {Milosav M. Marjanovi\'c and Aleksandra N. Mandi\'c},
     title = {Understanding algorithms of vertical addition and subtraction},
     journal = {The Teaching of Mathematics},
     pages = {57 },
     publisher = {mathdoc},
     volume = {XII},
     number = {2},
     year = {2009},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/TM2_2009_XII_2_a1/}
}