+#ifndef INTERACTION_H
+#define INTERACTION_H
+struct interaction_variable;
+struct interaction_value;
+
+struct interaction_variable * interaction_variable_create (const struct variable **, int);
+void interaction_variable_destroy (struct interaction_variable *);
+struct interaction_value * interaction_value_create (const struct interaction_variable *, const union value **);
+void interaction_value_destroy (struct interaction_value *);
+size_t interaction_variable_get_n_vars (const struct interaction_variable *);
+
+#endif