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Br Med J (Clin Res Ed) 1981; 283 doi: https://doi.org/10.1136/bmj.283.6307.1671 (Published 19 December 1981) Cite this as: Br Med J (Clin Res Ed) 1981;283:1671This is a PDF-only article. The first page of the PDF of this article appears above.