optionally, client data associated with them. Client data may be of two
different kinds: either simple untyped ({\tt void *}) pointers which are simply
stored by the control but not used in any way by it, or typed pointers
optionally, client data associated with them. Client data may be of two
different kinds: either simple untyped ({\tt void *}) pointers which are simply
stored by the control but not used in any way by it, or typed pointers