Checking for and adding "..." arguments to a function...

R-helpers:

I'm guessing this is an easy one for some of you, but I'm a bit
stumped.  Given some arbitrary function (doesn't matter what it does):

myfunction <- function(a,b,c)
{
return(a+b+c)
}

I want to test this function for the presence of the ellipses ("...")
and, if they are missing, create a new function that has them:

myfunction <- function(a,b,c,...)
{
return(a+b+c)
}

So, 1) how do I test for whether a function has an ellipses argument
and, 2) how do I "append" the ellipses to the argument list if they do
exist?

Note that the test/modification should be done without invoking the
function, e.g. I'm not asking how to test for this WITHIN the
function, I'm asking how to test "myfunction" directly as an R object.

Thanks!

--j