[Rd] External special functions (SPECIALSXP)

[Peter Danenberg]

> However, if you don't want to evaluate the arguments, just pass
> substitute(arg) to your function instead of arg.

Thanks, Duncan; the problem is, I'm trying to substitute on `...' and
I don't think I can access `...' without inadvertently evaluating it.

I'm trying to write a debugging function which, given any number of
expressions, prints the expression next to its evaluation.

This is trivial to do in the single arity case:

  debug <- function(expression) {
    cat(substitute(expression), expression, "\n")
  }

  a <- 2
  debug(a)

  a 2

but I'm not sure how to make it work with variable arity without
resorting to SPECIALSXP.