21.—(1) This rule applies where the prosecutor does not intend to call as a witness—
(a)any person whose statement or record of evidence has been served on the accused as part of the evidence for the prosecution; or
(b)any person in respect of whose evidence he has served notice under rule 56 below.
(2) Where this rule applies, unless the accused waives the requirement, the prosecutor shall—
(a)serve notice in writing on the accused that he does not intend to call that person; or
(b)tender that person at trial for cross-examination by the accused.