% A group around a single object is decoration: the stack already reads as
% one composite. The package must reject this with a dirty-build warning.
\documentclass[border=2pt]{standalone}
\usepackage[en]{supertensor}
\stsetrole{x}{stTeal}
\stdim{T}{4}
\begin{document}\begin{tikzpicture}
  \strow{R1}{T}
    \stgroup[role=x]{G}
      \ststack[role=x]{Q}{}{T}{T}{3}
    \stgroupend
  \strowend
\end{tikzpicture}\end{document}