The type of R's node released by GC
FREESXP.Rd
FREESXP is an object-like macro defining the SEXPTYPE
type of nodes released by garbage collector.
Declaration
#define FREESXP 31 /* node released by GC */
In Rinternals.h.
References
https://cran.r-project.org/doc/manuals/r-patched/R-ints.html#SEXPTYPEs