% \stlink inside a group: members never terminate a pending connector, so the
% arrow would be dropped silently. The package must error rather than build a
% figure whose label appears but whose arrow does not.
\documentclass[border=2pt]{standalone}
\usepackage[en]{supertensor}
\stsetrole{x}{stTeal}
\stdim{T}{4}
\begin{document}\begin{tikzpicture}
  \strow{R1}{T}
    \stgroup[role=x]{G}
      \stface[role=x]{A}{}{T}{T}
      \stlink{l1}{lost arrow}
      \stface[role=x]{B}{}{T}{T}
    \stgroupend
  \strowend
\end{tikzpicture}\end{document}