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Lawton, H. W.; Wilke, P. J. (1979). "Ancient Agricultural Systems in Dry Regions of the Old World". In Hall, A. E.; Cannell, G. H.; Lawton, H.W. (eds.). Agriculture in Semi-Arid Environments. Ecological Studies. Vol. 34 (reprint ed.). Berlin: Springer Science & Business Media (published 2012). p. 13. ISBN 9783642673283. Retrieved January 12, 2019.