\begin{frame}{Ambiguity Typically Unwanted}
    Ambiguity is typically unwanted:
      \item derivation trees often used to assign meaning to words,
      \item multiple derivation tree may result in double meaning.  

  In practice, ambiguity is often resolved outside of the grammar.
  For example, by a precedence on the rules: 
    \item For example, $0+2 * 1$ is parsed as $0+(2 * 1)$.

    \emph{Ambiguity is undecidable.}
    That is, there exists no algorithm that decides whether a context-free grammar is ambiguous.