\section{Child can specify its parents}

<<setup, include=FALSE>>=
set_parent('knitr-main.Rnw')
@

My parent is \textsf{knitr-main.Rnw}.

<<test-e>>=
1+1
@

I'm not a complete LaTeX document, but you can compile me nonetheless. I'll
borrow the preamble from my parent.