Perry (P), a voluntary fireman, was injured when he was struck by a fire truck as he was activating a pump manufactured and installed on the truck by Darley (D). D moved for an order to compel disclosure of the names of the experts who examined the pump and truck shortly after the accident. P conceded that he had a duty to disclose the identity of any expert whom he expected to call as a witness, (Rule 26(b)(1) but he refused to disclose the identity of those experts whom he did not expect to call as a witness (Rule 26(b)(4)).