\fbox{$\elabsto{e}{e}{\tau}$}

\begin{mathpar}

  \inferrule*[left=S-RefPass]{
    \elabsto{e_1}{\vectorvn}{\tarray}
    \\
    \elabsto{e_2}{i}{\tint}
    \\\\
    i \in \ints
    \\
    0 \le i < n
  }{
    \elabsto{\aref{e_1}{e_2}}{\unsaferef~\vectorvn~i}{\tint}
  }


  \inferrule*[left=S-RefFail]{
    \elabsto{e_1}{\vectorvn}{\tarray}
    \\
    \elabsto{e_2}{i}{\tint}
    \\\\
    i \in \ints
    \\
    i < 0 ~~\vee~~ i \ge n
  }{
    \elabsto{\aref{e_1}{e_2}}{\indexerror}{\tint}
  }

\end{mathpar}