Student's T Statistic
StudentsT(y, tr, tl, ...)
As Chung and Romano (2013) show, using the studentized difference of means as a permutation test statistic yields an asymptotically valid test of the "weak null" of mean equality between groups.
Chung, E. and Romano, J. P. (2013) Exact and asymptotically robust permutation tests. Annals of Statistics, 41, 484-507.