Historical clue formulations

• Preamble — 2022-06-19
• Show immediately preceding another — 2018-03-09
• Introduction — 2016-03-14
• Segue — 2012-07-27
• Prelim — 1999-11-13
• Opener — 1999-04-02
• Introduction — 1998-05-03
• Segue — 1997-12-26
• Opening — 1997-10-18
• Start of a segue — 1997-01-04