#include <math.h> for ceil, fmin, fmax, fabs, log10

[MichaelChirico]

We could also add these indirectly-included headers:

• <stdio.h> for snprintf
• <string.h> for strspn, strlen, strcat

These three (<math.h>, <stdio.h>, <string.h>) could replace <stdlib.h>.

We could also add specific headers for R:

• <R_ext/Arith.h> for NA_LOGICAL, NA_INTEGER, ISNA, ISNAN, R_FINITE, R_PosInf, R_NegInf
• <R_ext/Error.h> for error

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Feel free to request any of these changes, or to just close if transitive inclusion via <stdlib.h> is fine with you.