size_t design_matrix_get_n_rows (const struct design_matrix *);
double design_matrix_get_element (const struct design_matrix *, size_t, size_t);
void design_matrix_set_element (const struct design_matrix *, size_t, size_t, double);
size_t design_matrix_get_n_rows (const struct design_matrix *);
double design_matrix_get_element (const struct design_matrix *, size_t, size_t);
void design_matrix_set_element (const struct design_matrix *, size_t, size_t, double);