@article{SF_1991__43_3_135105,
     author = {B. Piochi},
     title = {Permutability of normal extensions of a semilattice.},
     journal = {Semigroup forum},
     pages = {151--162},
     publisher = {mathdoc},
     volume = {43},
     number = {3},
     year = {1991},
     zbl = {0737.20031},
     url = {http://geodesic.mathdoc.fr/item/SF_1991__43_3_135105/}
}