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
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.