From the NannyMUD documentation

LAST CHANGE

2001-01-06

FUNCTION

NAME
        is_armour - the object defining this function is a true armour


LOCATION
	/std/simple_armour, /obj/armour

SYNTAX
	int is_armour();

DESCRIPTION
	This function returns 1, and thus mark the object as a armour. It
	should be defined by objects that are true armours, i.e. that will
	handle all things a armour should. This means basically armour objects
	in the lib.


RETURN VALUES
	1 - the object is an armour