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Douglas K. Brown , Mariagnese Giusto , and Stephen G. Simpson , Vitali's theorem and WWKL, Archive for Mathematical Logic, vol. 41 (2002), no. 2, pp. 191–206.
Stephen G. Simpson , Subsystems of second order arithmetic, Perspectives in Mathematical Logic, Springer-Verlag, Berlin, 1999.
Xiaokang Yu and Stephen G. Simpson , Measure theory and weak Konig's lemma, Archive for Mathematical Logic, vol. 30 (1990), no. 3, pp. 171–180.