writing function with ,... )

Carmen -
   You certainly can write functions that use ..., but you need
to extract the arguments that the dots represent with list().
Here's a modified version of your function that may help explain
how this feature works.

test <- function(x,...){
    print(x)
    args = list(...)
    if('y' %in% names(args))print(args$y)
    if('z' %in% names(args))print(args$z)
}

                                       - Phil Spector
                     Statistical Computing Facility
                     Department of Statistics
                     UC Berkeley
                     spector at stat.berkeley.edu