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\begin{frame}
  \frametitle{Combinators}
  \vspace{-4ex}
  
  \begin{itemize}
    \item 
    Let $B = S(KS)K$.
  
    We have
    \begin{align*}
    Bxyz =   S(KS)Kxyz  
      &\pause\to KSx(Kx)yz\\
      &\pause\to S(Kx)yz \\
      &\pause\to Kxz(yz) \\
      &\pause\to x(yz)
    \end{align*}
  
  \pause
  \item
    Let $C = S(BBS)(KK)$. 
    
    We have 
    \begin{align*}
      Cxyz &\to^*  xzy
    \end{align*}
  \pause
  \item
    Exercise: find a combinator $F$ such that $Fxy = yx$.
  \end{itemize}
\end{frame}