@incollection{MEM_2021_50_a6,
     author = {Revalski, Julian},
     title = {Preface},
     booktitle = {},
     series = {Mathematics and Education in Mathematics},
     pages = {5--6},
     year = {2021},
     volume = {50},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/MEM_2021_50_a6/}
}