\documentclass[a4paper]{article}
\usepackage[latin1]{inputenc}
\begin{document}
Essayons de calculer 2 + 2 :
<<essai2plus2>>=
2 + 2
@

<<test, fig=F>>=
for(i in 1:10){
  print(paste("frame is", i))
  print(ls(envir = parent.frame(i)))
}
@ 
\end{document}