+ double (* func_f) (struct lmm_variable *var, double x); /* f */
+ double (* func_fp) (struct lmm_variable *var, double x); /* f' */
+ double (* func_fpi) (struct lmm_variable *var, double x); /* (f')^{-1} */
+ double (* func_fpip) (struct lmm_variable *var, double x); /* ((f')^{-1})' */