27/216
\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}