Several variations exist, with cosmetic variations such as differently named people, or variation in the crossing times or time limit.[5] The torch itself may expire in a short time and so serve as the time limit. In a variation called The Midnight Train, for example, person D needs 10 minutes instead of 8 to cross the bridge, and persons A, B, C and D, now called the four Gabrianni brothers, have 17 minutes to catch the midnight train.[1]The puzzle is known to have appeared as early as 1981, in the book Super Strategies For Puzzles and Games. In this version of the puzzle, A, B, C and D take 5, 10, 20, and 25 minutes, respectively, to cross, and the time limit is 60 minutes.[6][7] In all these variations, the structure and solution of the puzzle remain the same.In the case where there are an arbitrary number of people with arbitrary crossing times, and the capacity of the bridge remains equal to two people, the problem has been completely analyzed by graph-theoretic methods.[4]Martin Erwig from Oregon State University has used a variation of the problem to argue for the usability of the Haskell programming language over Prolog for solving search problems.[8]The puzzle is also mentioned in Daniel Dennett's book From Bacteria to Bach and Back as his favorite example of a solution that is counter-intuitive.