9.16 Arithmetic negation operator
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C-- lets you quickly specify arithmetic inversion of the sign of a
memory variable or register, by putting '-' before and ';' after its
identifier.
A few examples:
-AX; // same result as 'AX = -AX;' but quicker
-tree; // same as 'tree = -tree;' but quicker
-BH; // changes the sign of BH