\begin{Schunk}
\begin{Sinput}
 res <- persp(x=x1seq,y=x2seq, z=z, zlim=c(-3,3),theta=40, phi=-15)
 mypoints <- trans3d(dat$x1, dat$x2, dat$y, pmat = res)
 points(mypoints, pch =1, col="red")
\end{Sinput}
\end{Schunk}
