

   TThhee NNuullll OObbjjeecctt

        NULL
        as.null(x)
        is.null(x)

   DDeessccrriippttiioonn::

        `NULL' represents the null object in R.  NULL is used
        mainly to represent the lists with zero length, and is
        often returned by expressions and functions whose value
        is undefined.

        `as.null' ignores its argument and returns the value
        `NULL'.

        `is.null' returns `TRUE' if its argument is `NULL' and
        `FALSE' otherwise.

   EExxaammpplleess::

        is.null(list())    # TRUE (not so in S)
        is.null(integer(0))# F
        is.null(logical(0))# F
        as.null(list(a=1,b='c'))

