.BG
.FN levels
.TL
Levels Attribute
.CS
levels(x)
levels(x) <- value
.AG x
any object, typically a category.
.RT
if `x' has a `levels' attribute
then the value of this attribute is returned; otherwise `NULL'.
To explicitly delete the `levels' attribute,
use `levels(x) <- NULL'.
.KW attribute
.KW category
.WR
