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A general description is here.
Strict R headers are now the default. This removes the legacy definitions of ‘PI’, ‘Calloc’, ‘Realloc’ and ‘Free’: use ‘M_PI’, ‘R_Calloc’, ‘R_Realloc’ or ‘R_Free’ instead.
_Pro tem_ ‘STRICT_R_HEADERS’ is defined in header ‘R_ext/RS.h’ if not already defined. This is not used in R itself and will be removed before release.