如何在RStudio中调试（放置断点等）已安装的R包？

I need to run a function line-by-line to understand how it works. But the function is part of an installed package and I don't know where R stores the source of the installed packages (say MultiPhen). I am using RStudio 0.98.501 and R 3.0.2 in Ubuntu 12 (64it). Apparently source code of installed packages are not stored, right? Sorry if it is a naive question, I am new to R. If the sources are not stored, is there anyway to re-install a package with source and debug it (basically place a break point).I need to run a function line-by-line to unders