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==Swap-off Value==
(iii) The expected gain or loss if pieces en prise are captured. SOMA first calculates the "swap-off value" S for each piece, Black or White, which is en prise. S is a simple function of the value of the piece itself and of those which attack or defend it; it represents what the owner of the piece would lose if both players behaved efficiently.
Thus a White Knight defended by a Pawn and attacked by a Knight and a Bishop has S = 10 (White loses a Knight and Pawn for a Knight), whereas a White Pawn defended by a Pawn and attacked by a Knight has S = 0, because Black would not make the capture. S is necessarily zero or positive.
If only one Black piece en prise has a positive value S, White adds 5 to the value of his position. For suppose a Black Queen is attacked by a Pawn, it would be wrong to credit White with the full 90 points, since Black will almost certainly move his Queen, but White does score 5 for having the "initiative". If two or more Black pieces are en prise with positive value of S, White adds the second highest value of S, plus 5 for every other positive value; this supposes that Black will move the piece with the highest S.
For the White pieces en prise, SOMA subtracts from the value of his position the highest value of S, plus 5 for every other White piece with a positive value of S.
These rules concerning pieces en prise sound more complicated than they are. They ensure that SOMA will move or defend any White piece which Black can capture with advantage, that he will harry his opponent's pieces (+5 for initiative), and that he will fork his opponent of the opportunity arises, but they do not enable him to foresee a fork by his opponent. The effect of these rules in a more complex position is explained in the [[SOMA#note23|note]] to move 23 below.
==Misc==
