Guitart, R. (1980). Relations et carrés exacts. Ann. Sci. Math. Québec, 4(2), 103–125. 
@article{guitart:1980,
  title = {Relations et carr{\'e}s exacts},
  author = {Guitart, Ren{\'e}},
  year = {1980},
  journal = {Ann. Sci. Math. Qu{\'e}bec},
  volume = {4},
  number = {2},
  pages = {103--125}
}