@article{701516,
     title = {The tangent and the cotangent vector square of a pointed differential square},
     booktitle = {Proceedings of the Winter School "Geometry and Physics"},
     series = {GDML\_Books},
     publisher = {Circolo Matematico di Palermo},
     address = {Palermo},
     year = {1993},
     pages = {[163]-182},
     mrnumber = {MR1246631},
     zbl = {0796.58004},
     url = {http://dml.mathdoc.fr/item/701516}
}

Przybylski, Bronisław. The tangent and the cotangent vector square of a pointed differential square, dans Proceedings of the Winter School "Geometry and Physics", GDML_Books,  (1993), pp. [163]-182. http://gdmltest.u-ga.fr/item/701516/