@article{ARM_2006__42_4_a1,
     author = {Borceux, Francis and Bourn, Dominique and Johnstone, Peter},
     title = {Initial normal covers in {bi-Heyting} toposes},
     journal = {Archivum mathematicum},
     pages = {335--356},
     publisher = {mathdoc},
     volume = {42},
     number = {4},
     year = {2006},
     mrnumber = {2283017},
     zbl = {1164.18301},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/ARM_2006__42_4_a1/}
}