### This is almost like the Primitive ":" for factors
### (that has no "drop = TRUE") --- it's not used anywhere in "standard R"
interaction <- function(..., drop = FALSE, sep = ".")
{
    args <- list(...)
    narg <- length(args)
    if (narg == 1 && is.list(args[[1]])) {
	args <- args[[1]]
	narg <- length(args)
    }
    ans <- 0
    lvs <- NULL
    for(i in narg:1) {
        f <- args[[i]]
	if (!is.factor(f)) f <- factor(f)
	l <- levels(f)
	ans <- ans * length(l) + as.integer(f) - 1
	lvs <- if (i == narg) l	else as.vector(outer(l, lvs, paste, sep=sep))
    }
    ans <- structure(as.integer(ans+1), levels=lvs, class="factor")
    ans[ ,drop=drop]
}