\begin{Schunk}
\begin{Sinput}
  xf2 <- factor(xf2)
  table(x, xf2, exclude = NULL)
\end{Sinput}
\begin{Soutput}
      xf2
x      HECK YES <NA>
  F       0   0    1
  N       0   2    0
  P       0   0    1
  Y       2   0    0
  <NA>    0   0    0
\end{Soutput}
\end{Schunk}
