\begin{Schunk}
\begin{Sinput}
 merge(x = dat_legs, y = dat_fur, by = "animal", all = TRUE)
\end{Sinput}
\begin{Soutput}
  animal legs    fur
1   bird   NA     No
2   cats    4 Mostly
3    dog    4    yes
4  human    2     No
5  snake    0   <NA>
6   tree    0   <NA>
\end{Soutput}
\end{Schunk}
