get the bug context

Hi R-users,

How to get a bug contex? 

My R code ran there for several hours, but a bug crashed it, printing
such message like "Error: subscript out of bounds". I want to use
browser() to catch the bug, but I don't know which loop caused the bug
(there are many loops in the code). I even don't know which line of
code caused the bug. Is there any utility or something in R which can
let you know which line of code causes a bug?

Thanks,

-Luke