@article{SF_1982__24_134558,
     author = {H. Cohen and R.J- Koch},
     title = {Idempotent semigroups generated by threads {II.}},
     journal = {Semigroup forum},
     pages = {373--384},
     publisher = {mathdoc},
     volume = {24},
     year = {1982},
     zbl = {0504.22002},
     url = {http://geodesic.mathdoc.fr/item/SF_1982__24_134558/}
}