debugging a function in R that was not exported by a package

I would like to step through, using debug() or trace(), a function that was not exported. For example, how can I do it for vcov.polr of the package MASS, which is called from the function polr. polr is exported, but vcov.polr is not.

That is, when I run polr, I would like the debug browser to start once the code enters vcov.polr开发者_StackOverflow.

try

debug(MASS:::vcov.polr)

note that three colon ::: make the hidden object in a package visible.