name was supplied
as an argument to the function in which
missing is placed.
missing(name)
missing occurs.
TRUE
if the current call did not supply the argument named,
FALSE
if it did.
It is an error if the argument to
missing is not the name
of an argument.
missing
allows arguments to be omitted even though no default
value is supplied (provided that some appropriate action is taken before
the value of the argument is needed).
Used internally within a function as a conditional. Checks to see
if the
name argument was supplied to the function.
function(y, weight)
{
if(!missing(weight))y <- y * sqrt(weight)
...
}