void covariance_destroy (struct covariance *cov);
-const gsl_matrix *covariance_moments (const struct covariance *cov, int m);
+gsl_matrix *covariance_moments (const struct covariance *cov, int m);
const struct categoricals * covariance_get_categoricals (const struct covariance *cov);
size_t covariance_dim (const struct covariance * cov);