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lepton is one of three charged leptons in nature, the measurements of its mass have been performed since its discovery. The present relative accuracy is already at the level of ; more factors are still being studied in order to increase the accuracy. However, the available techniques for analysis and expectable luminosity from e e collider indicate that the precision upper limit of mass is almost reached, which means that brand new approaches should be considered if a great improvement is yearned for. [ABSTRACT FROM AUTHOR]