## S3 method for class 'Categories':
is.na(x)## S3 method for class 'Categories,character':
is.na(x) <- value
## S3 method for class 'Categories,logical':
is.na(x) <- value
## S3 method for class 'Category':
is.na(x)
## S3 method for class 'Category,logical':
is.na(x) <- value
x
duly modified.