\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} \pause\medskip 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} \pause\medskip \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}