Missing or NULL class union
Examples
showClass("missingOrNULL")
#> Virtual Class "missingOrNULL" [package "AcidBase"]
#>
#> No Slots, prototype of class "NULL"
#>
#> Known Subclasses:
#> Class "NULL", directly
#> Class "missing", directly
#> Class ".NULL", by class "NULL", distance 2, with explicit coerce