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\begin{frame}{Ambiguity Typically Unwanted}
\begin{goal}{}
Ambiguity is typically unwanted:
\begin{itemize}
\item derivation trees often used to assign meaning to words,
\item multiple derivation tree may result in double meaning.
\end{itemize}
\end{goal}
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In practice, ambiguity is often resolved outside of the grammar.
\smallskip

For example, by a precedence on the rules:
\begin{itemize}
\item For example, $0+2 * 1$ is parsed as $0+(2 * 1)$.
\end{itemize}
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\begin{block}{}
\emph{Ambiguity is undecidable.}
That is, there exists no algorithm that decides whether a context-free grammar is ambiguous.
\end{block}
\end{frame}