\begin{Schunk}
\begin{Sinput}
 V <- vcov(m2)
 varsum <- V[2,2]+mean(dat$x2)^2*V[4,4]+2*mean(dat$x2)*V[2,4]
 varsum
\end{Sinput}
\begin{Soutput}
[1] 1.895999
\end{Soutput}
\begin{Sinput}
 sqrt(varsum)
\end{Sinput}
\begin{Soutput}
[1] 1.376953
\end{Soutput}
\end{Schunk}
