void interaction_value_destroy (struct interaction_value *);
size_t interaction_variable_get_n_vars (const struct interaction_variable *);
double interaction_value_get_nonzero_entry (const struct interaction_value *);
-union value * interaction_value_get (const struct interaction_value *);
+const union value *interaction_value_get (const struct interaction_value *);
const struct variable * interaction_variable_get_var (const struct interaction_variable *);
size_t interaction_get_n_numeric (const struct interaction_variable *);
size_t interaction_get_n_alpha (const struct interaction_variable *);