This class is only used when user_na = TRUE in
read_sav(). It is similar to the labelled class
but it also models SPSS's user-defined missings, which can be up to
three distinct values, or for numeric vectors a range.
A vector to label. Must be either numeric (integer or double) or
character.
labels
A named vector. The vector should be the same type as
x. Unlike factors, labels don't need to be exhaustive: only a fraction
of the values might be labelled.
na_values
A vector of values that should also be considered as missing.
na_range
A numeric vector of length two giving the (inclusive) extents
of the range. Use -Inf and Inf if you want the range to be
open ended.