Boot, M.
Servire Katwijk , 1984
Buku Teks
Neijt, Anneke
Foris Publications, 1990
Buku Teks
edited by Istvan Batori, Winfried Lenders, Wolfgang Putschke
Walter de Gruyter, 1989
Buku Referensi
Longman , 1995
Buku Teks
Cambridge University Press, 2008
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