`missing(x)`

is only reliable if `x`

has not been altered
since entering the function: in particular it will *always*
be false after `x <- match.arg(x)`

.

The example shows how a plotting function can be written to work with
either a pair of vectors giving x and y coordinates of points to be
plotted or a single vector giving y values to be plotted against their
indices.

Currently `missing`

can only be used in the immediate body of
the function that defines the argument, not in the body of a nested
function or a `local`

call. This may change in the future.

This is a ‘special’ primitive function: it must not
evaluate its argument.