double expr_evaluate_num (struct expression *, const struct ccase *,
int case_idx);
void expr_evaluate_str (struct expression *, const struct ccase *,
int case_idx, char *dst, size_t dst_size);
double expr_evaluate_num (struct expression *, const struct ccase *,
int case_idx);
void expr_evaluate_str (struct expression *, const struct ccase *,
int case_idx, char *dst, size_t dst_size);