[NTG-context] Mixed subscripts and superscripts

I'm trying to do this https://en.wikipedia.org/wiki/Mixed_tensor doing this \m{T_{\alpha}^{\beta}_{\gamma}} and this \m{T^{\alpha}_{\beta}^{\gamma}} but I get errors for double subscript and double superscript respectively. Then I'm doing this as a workaround T_{\alpha~\gamma}^{~\beta~} But the ~ has the fixed width of a digit, or so I read, and I get a less optimal spacing in this case \m{T^{i~k~m}_{~j~l~n}} in which the 'm' appears to be after the 2014-05-21_22:04 on debian testing