object Person {
	/**
	 * Accessors are compact. Invariants can use assertions for validation.
	 * Unless otherwise qualified, the body defines the setter characteristics only.
	 */
	accessor notnull int age {
	 	assert $ > 0 : "Must be greater than 0";
	};

	/**
	 * The := operator overrides control and sets the value according the RHS expression.
	 */
	accessor notnull String name {
		assert notempty $ : "Must have a value";
		:= $.toUpperCase();
	};

	/**
	 * Regular constructor, still supported.
	 */
	def (String name, int age) {
		this.name = name;
		this.age = age;
	}

	/**
	 *
	 */
	accessor Person friend {
		set { assert $ != null : "Cannot reference self" }
		get { friend.evil ? null : friend }
	};

	def sayHello : String {
		"Hello, my name is " + name;
	}
}

person = Person.new::{ age = 30, name = "Mark" };

/**
 *  Throws an exception: name cannot be null.
 */
person = Person.new::{ age = 20 };

/**
 * Regular constructor can still be called
 */
person = new Person("Mark", 30);