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426. Andrej Lobusov
Chess in Australia 1981
Mate in 2

The black king has a flight-move to e5, answered by a triple, 2.Sb7/Sc4/Sf7 (curiously these mates all recur individually in subsequent play). Both the try and the key by the e5-rook remove that flight but grant a new one, besides activating the half-battery on the dark diagonal. The thematic try 1.Rxe3? threatens 2.Qf2 and leads to 1…Kxe3 2.Sdf5, 1…dxe3 2.Sb7, 1…Ra1 2.Sc4, and 1…Sxd3/Se4/Sxe6 2.Re4. But 1…g4! refutes, as the set mate 2.Rf5 for this defence is no longer possible. The key 1.Rxg5!, with the threat of 2.Qxh4, brings about some attractive changes relative to the try play: 1…Kxg5 2.Sf7 (new flight and mate), 1…Sxg5 2.Sb7 (transferred mate), and 1…Ra1 2.Se4 (changed mate).

Andy Sag: The half battery gives a strong clue to the give-and-take key. Very nice changes!
Jacob Hoover: There are three defences after the key, and each one forces the B + S battery to fire in a different way. A proliferation of battery plays (one of which is indirect) and no distracting by-play.
Ian Shanahan: A lovely sacrificial, flight-giving key from the half-battery. There is a symmetrical try and threat, defeated by 1…g4! Grandmaster Lobusov was working on spectacular keys and battery play at the time.

427. Gordon Stuart Green
The Problemist 1970
Jubilee Tourney
1st Prize
Mate in 3

If Black were to begin, every legal move has an immediate mating response set: 1…f2 2.Se2, 1…S~ 2.Qe5, and 1…B~ 2.Qf2/Qh2. With no waiting move that could maintain all of these variations, White must play a key that extends the solution to three moves, making this an example of a pseudo two-mover. The excellent key 1.Sh3! changes the replies to two black defences and also adds two more variations, by granting a flight on h4 and sacrificing the knight. After 1…f2, White answers with the threat-move 2.Sxf2, which leaves Black in zugzwang: 2…B~ 3.Rfg1, 2…Bxe4!? (correction move) 3.Sxe4, and 2…S~ 3.Qe5. Since 2.Sxf2! acts like the key of a two-mover, generating three precise variations, this problem also exemplifies an insert two-mover. The second change is 1…S~ 2.Sf4 and 3.Sxh5, or 1…Sf6 2.Sf4 (waiting) 2…S~ 3.Sxh5, 2…f2 3.Se2, 2…B~ 3.Qf2/Qh2/Rfg1. Black taking the flight leads to 1…Kxh4 2.Sf4+ Kg5 3.Rxh5, 2…Kg3 3.Sxh5. Or if Black accepts the sacrifice: 1…gxh3 2.Rhg1+ Kxh4/Kh2 3.Sxf3. Lastly, the set dual is removed in 1…B~ 2.Qf2. This difficult problem was placed first in a tourney for three-movers that are either pseudo or insert two-movers, but it manages to combine the two ideas.

Andy Sag: Sacrificial flight-giving key with a waiting move threat. First time I've seen that!
George Meldrum: Amazing.

428. Abdelaziz Onkoud
Australian Chess Problem Magazine 1995
Mate in 2

Set mates are prepared for most of Black’s moves in the diagram, except for 1…dxe6 and 1…Se2. The key 1.Bd3! (waiting) completes the block by providing for these two defences and it also adds two variations by unpinning the d4-pawn. If this pawn captures either rook, White exploits the opened d-file: 1…dxe3 2.Bf5 and 1…dxc3 2.Bb5. The key-bishop mates directly after the self-blocks, 1…dxe6 2.Be4 and 1…dxc6 2.Bc4. The remaining defences are mostly unguards: 1…f5 2.Re5, 1…b5 2.Rc5, 1…cxd6 (self-block) 2.Sxb6, 1…S8~ 2.Sxf6, 1…a3 2.Qb3, 1…Sf3 2.Qxf3, and 1…Se2 (line-opening) 2.Qh1. A total of eleven variations, including eight pawn defences.

Andy Sag: The key unpins the d4-pawn, removes the set mate 1…d3 2.Qxd3 but adds four mates making eleven variations including five symmetrical pairs, the highlight being the pair of battery mates after each rook is captured!
Jacob Hoover: Tries 1.Be2/Bb5/Ba6? dxe6! and 1.e7/exd7? Se2! An easy but nonetheless rewarding solve, as the variations display multiple themes (battery plays, self-blocks enabling white self-interference).
George Meldrum: Eleven ways to checkmate, and although many are in set play the new added mates are outstanding.
Ian Shanahan: An incomplete-block. 1…Se2 has no mate set for it, which points to the key piece (if not its destination). A waiter in ye olde style, with a fine sacrificial key that eliminates the set variation 1…d3 2.Qxd3.

429. Henryk Grudziński
Australian Chess 2007
Helpmate in 2
Twin (b) Ka4 to d6

The first part is solved by 1.Rb2 Rf3 2.Sb4 Bxb5. The black rook unblocks a flight in order to unguard the eventual mating square b5 (without closing prospective white lines: 1.Rc4/Rb3?). The white rook prepares an attack behind the d3-knight, which then opens lines simultaneously for both white pieces. The knight also blocks the square that was vacated by the black rook and interferes with the latter’s control of b5. The white rook is thus allowed to guard two flights while the white bishop traverses the opened line to give mate. The second part has the black king starting on d6, which changes the solution to 1.e5 Bg4 2.Se7 Rxf6. Now the black pawn creates a flight on e7 and unguards the mating square f6, while also self-blocking e5. After the white bishop is placed behind the f5-knight, the latter opens lines for both white pieces and blocks the square vacated by the e-pawn, besides cutting off the e8-rook. With the white bishop activated to observe the d7-flight, the white rook mates by moving along the opened line.

Jacob Hoover: In each part a black unit moves in order to enable a black knight to perform a necessary self-interference; also, the white rook and bishop exchange functions between the two parts.
George Meldrum: A single black knight move plugs a flight square, blocks a defence, and opens lines for White’s bishop and rook, in both versions.
Andy Sag: Rook and bishop take turns in guarding escape squares while the other piece mates, after a knight clears lines as well as self-blocking. The f6-pawn appears to be unnecessary apart from ensuring the mate involves a pawn capture.
Nigel Nettheim: The exchange of roles of the two white pieces is nice, although only about half the black units take part in each twin. Easy to solve.
Michael McDowell: What is the idea of the problem? A black knight replaces an ineffective self-blocker, while the white rook and bishop exchange functions, taking turns to guard flights by ambushing behind a knight, or mate by moving along a vacated line. Unfortunately the strategy is mismatched. The only task of the b4 rook is to unguard b5, and it chooses b2 to avoid closing a line which White needs open. The e7-pawn must unguard f6, but e5 must be blocked, so its move has a second purpose. The self-block 2.Sb4 happens to interfere with the b2-rook, so the composer added a rook at e8 to create the impression of matching interferences. Helpmate composers call this sort of piece a weasel – the rook is superfluous and is unnecessary for soundness. Because a pawn at b5 is needed to shield the white king the composer added a pawn at f6 simply in order to have matching captures on the mating moves. This pawn is also superfluous. A lighter and more harmonious problem could be created by replacing the black square vacations with self-blocking moves which move across the squares which the knights will then block.

Michael McDowell
(after H. Grudziński)
OzProblems.com 2019
Helpmate in 2
Twin (b) Ka4 to c1

(a) 1.Ba3 Re5 2.Sb4 Bb3.
(b) 1.Rb2 Bh7 2.Sd2 Re1.