\begin{frame}{Words}

  \begin{block}{}
    \emph{Word} = 
    finite sequence of \emph{symbols} %($a,b,c,\ldots$) 
    from an \emph{alphabet $\Sigma$}.
  \end{block}  
  \begin{itemize}
    \item notation for symbols: $a,b,c,\ldots$
    \item notation for words: $u,v,w,x,y,z$
    \item $a\in\Sigma$ means $a$ is a symbol from the alphabet $\Sigma$
  \end{itemize}
  \pause\bigskip
  
  \begin{block}{}
    We write $\lambda$ for the \emph{empty word}.
  \end{block}
  \pause
  \alert{\emph{Important: $\boldsymbol{\lambda}$ is not a letter of the alphabet\,!}}
  \pause\bigskip
  
  \begin{goal}{}
    In programming, words are called \emph{strings}.
    \medskip
    
    Then $\lambda$ is the empty string $"\,"$ (has length $0$).
  \end{goal}
\end{frame}