Sociologist Rodney Stark states that Jehovah's Witness leaders are "not always very democratic" and that members "are expected to conform to rather strict standards," but adds that "enforcement tends to be very informal, sustained by the close bonds of friendship within the group", and that Jehovah's Witnesses see themselves as "part of the power structure rather than subject to it. What statement does Stark make about the leaders of the Jehovah's Witnesses?