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\begin{frame}
\small

\begin{example}
$t = \GREEN{(2:x)+((1:x):y)}$ \qquad
\GREEN{\begin{tikzpicture}[on grid,baseline=(1).baseline]
\matrix[ampersand replacement=\&,row sep=5mm,column sep=2mm]{
\& \& \& \node (1) {$+$}; \& \& \& \\
\& \node (2) {$:$}; \& \& \& \& \node (3) {$:$}; \& \\
\node (4) {$2$}; \& \& \node (5) {$x$}; \& \& \node (6) {$:$}; \& \&
\node (7) {$y$}; \\
\& \& \& \node (8) {$1$}; \& \& \node (9) {$x$}; \& \\};
\draw (1) -- (2) (1) -- (3);
\draw (2) -- (4) (2) -- (5);
\draw (3) -- (6) (3) -- (7);
\draw (6) -- (8) (6) -- (9);
\draw (1) node[right] {$\,{}^\epsilon$};
\draw (2) node[left] {${}^1\,$};
\draw (3) node[right] {\alert<7>{$\,{}^2$}};
\draw (4) node[left] {${}^{11}\,$};
\draw (5) node[right] {$\,{}^{12}$};
\draw (6) node[left] {\alert<3>{${}^{21}\,$}};
\draw (7) node[right] {$\,{}^{22}$};
\draw (8) node[left] {${}^{211}\,$};
\draw (9) node[right] {\alert<5>{$\,{}^{212}$}};
\end{tikzpicture}}
\begin{itemize}
\item<2->
$t|_{21} = \alt<3->{\GREEN{1:x}}{?}$
\item<4->
$t(212) = \alt<5->{\GREEN{x}}{?}$
\item<6->
$t[\GREEN{x+3}]_{2} = \alt<7->{\GREEN{(2:x)+(x+3)}}{?}$
\smallskip
\end{itemize}
\end{example}

\end{frame}