\begin{Schunk}
\begin{Sinput}
  xf3 <- xf2
  levels(xf3) <- c(levels(xf3), "Denver")
  xf3[5] <- "Denver"
  data.frame(x, xf1, xf2, xf3)
\end{Sinput}
\begin{Soutput}
  x xf1  xf2    xf3
1 P   N HECK   HECK
2 N   N  YES    YES
3 Y   Y HECK   HECK
4 Y   Y HECK   HECK
5 F   F <NA> Denver
6 N   N  YES    YES
\end{Soutput}
\end{Schunk}
