[R] Creating NA equivalent

[Duncan Murdoch]

On 20/12/2021 11:41 p.m., Marc Girondot via R-help wrote:
> Dear members,
> 
> I work about dosage and some values are bellow the detection limit. I
> would like create new "numbers" like LDL (to represent lower than
> detection limit) and UDL (upper the detection limit) that behave like
> NA, with the possibility to test them using for example is.LDL() or
> is.UDL().
> 
> Note that NA is not the same than LDL or UDL: NA represent missing data.
> Here the data is available as LDL or UDL.
> 
> NA is built in R language very deep... any option to create new version
> of NA-equivalent ?
> 

There was a discussion of this back in May.  Here's a link to one 
approach that I suggested:

   https://stat.ethz.ch/pipermail/r-devel/2021-May/080776.html

Read the followup messages, I made at least one suggested improvement. 
I don't know if anyone has packaged this, but there's a later version of 
the code here:

   https://stackoverflow.com/a/69179441/2554330

Duncan Murdoch