The type of R's NULL
NILSXP.Rd
NILSXP is an object-like macro defining the SEXPTYPE
type of a SEXPREC object pointed by R_NilValue
(the R level NULL object).
Declaration
#define NILSXP 0 /* nil = NULL */
In Rinternals.h.
References
https://cran.r-project.org/doc/manuals/r-patched/R-ints.html#SEXPTYPEs