"The first safeguard is the power of control by the trial judge. It is a preliminary question for his decision whether the memorandum actually does refresh and from the nature of the memorandum and the witness' testimony he may find that it does not. Moreover, as in the case of leading questions, he may decline to permit the use of the aid to memory, where he regards the danger of undue suggestion as outweighing the probable value, in the exercise of his discretion to control the manner of the examination. The second safeguard is the rule which entitles the adverse party, when the witness seeks to resort to the memorandum, to inspect the memorandum so that he may object to its use if ground appears, and to have the memorandum available for his reference in cross-examining the witness. With the memorandum before him, the cross-examiner has a good opportunity to test the credibility of the witness' claim that his memory has been revived, and to search out any discrepancies between the writing and the testimony. This right to inspection has usually been limited to writings used by the witness on the stand, but the reasons seem equally applicable to writings used by the witness to refresh his memory before he testifies. Doubtless the courts have thought that to require inspection of such papers may unduly encourage prying into the opponent's file, but the public interest in the full disclosure of the source of a witness's testimony seems a weightier consideration, and there are decisions taking this view."