Debugging generic functions in R

How do you debug a generic function (using debug, or mtrace in the debug package)?

As an example, I want to debug cenreg in the NADA package, specifically the method that takes a formula input.

You can retrieve the method details like this:

library(NADA)
getMethod("cenreg", c("formula", "missing", "missing"))

function (obs, censored, groups, ...) 
{
    .local <- function (obs, censored, groups, dist, conf.int = 0.95, 
        ...) 
    {
        dist = ifelse(missing(dist), "lognormal", dist)

...
}

The problem is that cenreg itself looks like this:

body(cenreg)
# standardGeneric("cenreg")

I don't know how to step through the underlying method, rather than the generic wrapper.