Historical clue formulations

• Extremely complicated, as a problem — 2025-03-18
• Tough — 2021-04-03
• Difficult — 2015-05-06
• Tough to figure out — 2014-08-01
• Gnarled, as a tree trunk — 2012-01-09
• Labyrinthine — 2011-09-04
• Full of difficulties — 2010-04-14
• Intricate — 2007-03-03
• Stubborn, as a problem — 2002-10-24
• Problematic — 2000-03-26
• Hard to saw, as some pine — 2000-03-06
• Complex — 1995-04-06