[Rd] good practice for values not provided

[Tamas K Papp]

Hi,

I am writing a collection of functions which I plan to share as a
package later (when they are tested thoroughly), so I would like to do
things "right" from the beginning...

I encountered a minor question of style.  Consider a function 

f <- function(a,b,x=NULL) {
  ## ...
}

if !is.null(x), f will use x to calculate the result, but if
is.null(x), it will do something else not involving x at all (using
any x would be meaningless here, so I can't use
x=calcsomethingfrom(a,b)).

What's the accepted way of indicating this in R with a default for x?
x=FALSE? x=NA? x=NULL?

Thanks,

Tamas