Help for Exocet Pattern
 

This pattern, known variously as Exocet, jExocet, bi bi, BB is recognised as a pattern that frequently is present in very hard puzzles. When 2 of the 3 cells in a box-line intersection together contain 3 or 4 candidates, then in each of the two boxes in the same band but in different lines, if there are cells with the same 3 or 4 candidates, any others can be removed. The conditions that have to be met for it to be a valid exocet are described in David bird's thread. I won't attempt to discuss the workings of the exocet pattern as David's Compendium provide's great detail. I have not yet implemented all of David's techniques. This is an early example by champagne :

.......39.8......5..9.6.8....5.9...67....2......4.......3.8..5..2.7..6..4.....1.. colx062,coloin 2.8 3BB r12c7 r4c8 r7c9

Beware of puzzles like these:

........4....3..2.41...27..1..36..8..7.1...56....58....4...3.....2.....93..5...6.
.......39...3.1..5....5.8....8.9...6.7...23..1..4......69.8..5..21...6..4..7.6...
which at first glance appear to have an Exocet patterns, but don't fullfill the basic requirements, and this leads to invalid eliminations.

Double Exocets

When two exocets occur in the same chute involving the same 4 numbers, a special situation occurs and extra eliminations may be possible. This was described by ttt (see Fri Jan 28, 2011 2:40 pm. This functionality has also been added. ttt's example is the first, the second is Flockman's, and the last two among those posted by eleven Fri Jan 28, 2011 9:21 am:

..3......4...8..36..8...1...4..6..73...9..........2..5..4.7..686........7..6..5..
6....894.9....61...7..4....2..61..........2...89..2.......6...5.......3.8....16..
...4......5..8.2.6.....71..2...4....3......1...5.3.8.25...6.3.8..6....9...8......
1...5.7.9..71......6.......2...........5.1..2....2.39.3...9...15...1...3...8...4.
98.76.5..7.4.......5..8.9..3....6....9..5.6.........2..1...5..6...6.81......1..59
..............1..2..2.3..45.2..4..633..7.....4....38...5......8.6...9.5.2...5..36
...4....87.......9..3.9.5....9.6...51....23...7....6....6.8..5......1....2...4...