

   DDooeess aa FFoorrmmaall AArrgguummeenntt hhaavvee aa VVaalluuee??

        missing(x)

   DDeessccrriippttiioonn::

        `missing' can be used to test whether a value was spec-
        ified as an argument to a function.  The following
        example shows how a plotting function can be written to
        work with either a pair of vectors giving x and y coor-
        dinates of points to be plotted or a single vector giv-
        ing y values to be plotted against their indexes.

   SSeeee AAllssoo::

        `NA' for ``missing values'' in data.

   EExxaammpplleess::

        myplot <- function(x,y) {
                        if(missing(y)) {
                                y <- x
                                x <- 1:length(y)
                        }
                        plot(x,y)
                }

