class.c:220
VALUE
rb_define_class_under(outer, name, super)
    VALUE outer;
    const char *name;
    VALUE super;
{
    VALUE klass;
    ID id;

    id = rb_intern(name);
    if (rb_const_defined_at(outer, id)) {
	klass = rb_const_get_at(outer, id);
	if (TYPE(klass) != T_CLASS) {
	    rb_raise(rb_eTypeError, "%s is not a class", name);
	}
	if (rb_class_real(RCLASS(klass)->super) != super) {
	    rb_name_error(id, "%s is already defined", name);
	}
	return klass;
    }
    if (!super) {
	rb_warn("no super class for `%s::%s', Object assumed",
		rb_class2name(outer), name);
    }
    klass = rb_define_class_id(id, super);
    rb_set_class_path(klass, outer, name);
    rb_const_set(outer, id, klass);
    rb_class_inherited(super, klass);

    return klass;
}
