Add axiom to ruleset

include/fractal.hpp

@@ -100,7 +100,10 @@ private:
 
 	etl::vector<branch_rule_t, g_max_branch_rules> m_branch_rules;
 
+	group_id_t m_axiom = 0;
+
 public:
+
 	/**
 	 * @brief Checks if the maximum number of tokens has been reached
 	 *
@@ -219,6 +222,14 @@ public:
 	 */
 	branch_rule_id_t add_branch_rule_weighted(token_id_t match, weighted_group_id_t wgroup);
 
+	/** @brief Sets the axiom
+	 *
+	 * @exception @c mtl::invalid_argument if an invalid group ID was supplied
+	 */
+	void set_axiom(group_id_t axiom) { 
+		m_axiom = valid_group_characteristic(axiom) ? axiom : throw mtl::invalid_argument();
+	}
+	group_id_t get_axiom() const { return m_axiom; }
 
 };