[R] passing arguments via "..."

[Cable, Samuel B Civ USAF AFMC AFRL/RVBXI]

I would like to do something like the following:

Fancyhist<-function(x,...) {

# first, process x into xprocess somehow, then ...

  if (is.null(breaks)) {   # yes, I know this is wrong
# define the histogram breaks somehow, then call hist:
     hist(xprocess,breaks=breaks,...)
  } else {
# use breaks give in calling argument
     hist(xprocess,...)
  }

}

But, those of you who know R better than I do have already spotted that
"is.null(breaks)" just won't work.  Neither does "exists" or "missing".

I can think of ways to do this involving new arguments with new names --
like "histbreaks" or something -- but that seems inelegant.

Is there a way to do what I am trying to do here?

Thanks.