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A general description is here.
Header ‘R_ext/Constants.h’ (included by ‘R.h’) now includes header ‘float.h’ or ‘cfloat’ for constants such as ‘DBL_MAX’ even if ‘STRICT_R_HEADERS’ is defined.
The legecy definition of ‘PI’ in ‘R_ext/Constants.h’ (included by ‘R.h’) will be removeed shortly: use ‘M_PI’ instead.