I am trying to obtain a black square with a subscript and a superscript in plain TeX. So far I managed to create
$\buildrel abc \over\square$.
I could easily use nested
\buildrels to achieve the desired result, but then, the font used for superscript is smaller than the one for the subscript, because what TeX sees is a "superscript" above
\square above "subscript".
How can I solve this? For example, how does TeX implement the
\sum macro with superscript and subscript that are equally scaled?