@article{DMGT_2001_21_2_a5,
     author = {Haviar, Alfonz and Monoszov\'a, Gabriela},
     title = {On varieties of orgraphs},
     journal = {Discussiones Mathematicae. Graph Theory},
     pages = {207--221},
     publisher = {mathdoc},
     volume = {21},
     number = {2},
     year = {2001},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/DMGT_2001_21_2_a5/}
}