sec. 1.1 says: "both types of node structure have as their first three fields a 32-bit sxpinfo header and then three pointers [...]" that's *four* fields, as seen in src/include/Rinternals.h:208+: #define SEXPREC_HEADER \ struct sxpinfo_struct sxpinfo; \ struct SEXPREC *attrib; \ struct SEXPREC *gengc_next_node, *gengc_prev_node vQ