[texhax] apostrophe/transpose with subscripts

[Robert Duval]

Dear All

I want to write a formula of the style X'_nX_n. (note the
transpose/apostrophe after the first X)

However, as i type this in math mode i get that the subscript n of the
first X lies below the one of the second X.

I want both subscripts to be at the same height.

I could do it by implementing either

{X'}_n or {X_n}'

but the look of these is not what i want. I really would like the
apostrophe above the _n and both subscripts to be at the same height.

Any help on how to achieve this would be greatly appreciated

thanks.
robert