\\
\begin{rail} \\ \end{rail}

x \\
\begin{rail} x \\ \end{rail}

\\ x
\begin{rail} x \\ \end{rail}

x \\ y
\begin{rail} x \\ y \end{rail}

\\ | x \\ | \\ y | x \\ y
\begin{rail} \\ Vert x \\ Vert \\ y \vert x \\ y \end{rail}

x +
\begin{rail} x + \end{rail}

\\ +
\begin{rail} \\ + \end{rail}

\\ + \\
\begin{rail} \\ + \end{rail}

\\ *
\begin{rail} \\ * \end{rail}

\\ * \\
\begin{rail} \\ * \end{rail}

\\ ?
\begin{rail} \\ ? \end{rail}

(x *)
\begin{rail} x * \end{rail}