allNA tests if all elements are NA, and
includeNA sets the
NAs in a character vector or factor to an explicit label.
allNA(x) includeNA(x, label, ...) # S3 method for factor includeNA(x, label = "(Missing)", first = FALSE, ...) # S3 method for default includeNA(x, label = "(Missing)", ...)
A character string denoting the label to set
Other arguments (not in use at this time).
Logical; should the new label be the first level?