@article{CTGDC_1977__18_4_409_0,
     author = {Spencer, Christopher B.},
     title = {An abstract setting for homotopy pushouts and pullbacks},
     journal = {Cahiers de topologie et g\'eom\'etrie diff\'erentielle},
     pages = {409--429},
     publisher = {Dunod \'editeur, publi\'e avec le concours du CNRS},
     volume = {18},
     number = {4},
     year = {1977},
     mrnumber = {486054},
     zbl = {0378.18008},
     language = {en},
     url = {http://www.numdam.org/item/CTGDC_1977__18_4_409_0/}
}