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3. Bc4 Qh4+

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The package is hefty, covered in crinkled brown wrappings and obviously conceals a large book. There is a note attached: Do not upset Mummy. Merry Christmas.

Sherlock frowns. Judging by previous calculations, the parcel should have taken a week to arrive, but only three days had passed since he'd had his last confrontation with Mycroft. But Mycroft's burgeoning influence is not Sherlock's problem right now - his mind is restless, and Sherlock hates to be bored. He peels off the packaging in layers, sweeps the mess on the floor and turns to examine his present.

It is a single, massive slab, easily three inches thick and bound in a hard black cover: Chess: 5334 Problems, Combinations Games. It trembles in his pale arms (at least five and a half pounds, thinks Sherlock). He carries it delicately to his room and drops it on the floor with a loud thud.

Settling down next to it, Sherlock fingers the spine, and reads.

The pages turn with a wonderfully crisp sound, and he drinks in the mate-in-ones like warm tea. They slip into his mind easily, three hundred and five newly acquired weaponry for use on the board. Except these knives are too crude or too thin and too easy to mangle with and he almost deletes them out of sheer spite for Mycroft and his boredom inducing ways, until he finds, grudgingly, that attempts to do so simply do not work; the pattern sings and captures and resonates within them all, crude mate-in-ones or not.

It therefore comes as a shock and an utter delight when the three hundred and sixth puzzle makes him think for all of fifty seconds.

He stares at it for ages before understanding: When you have eliminated the impossible, whatever remains, however improbable, must be the truth. There is no piece that can deliver mate except for the rook; the rook is blocked by the king; the king cannot move except to castle. The answer is castling.

Sherlock laugh is high and clear. Number three hundred and seven is easy now - the ridiculous answer made simple by pure logic, an en passant that finishes the collection. The pattern glows in praise. Sherlock keeps the knowledge of these two knives carefully, and moves on.

At the party the next week, Sherlock gracefully accedes to Mummy's request that he play the violin. His mind is filled with music and darting pieces, patterns, rhythm, and when the mail brings three new books a week later, Sherlock is already using the first as a chair.