\begin{Schunk}
\begin{Sinput}
 dat$x1c <- dat$x1 - mean(dat$x1, na.rm = TRUE)
 dat$x2c <- dat$x2 - mean(dat$x2, na.rm = TRUE)
 m3 <- lm(y ~ x1c * x2c, data=dat)
\end{Sinput}
\end{Schunk}
