@article{AMLG_1972__15_137879,
     author = {Robert Fittler},
     title = {Some categories of models.},
     journal = {Archiv f\"ur mathematische Logik und Grundlagenforschung},
     pages = {179--190},
     publisher = {mathdoc},
     volume = {15},
     year = {1972},
     zbl = {0272.02072},
     url = {http://geodesic.mathdoc.fr/item/AMLG_1972__15_137879/}
}