= Sort key
{title2=$k(A_i)$}
A <sort key> is the value extracted from a complete entry to determine its ordering. Model extraction as a <function> $k$ into a <total order>; compare $k(A_i)$ and $k(A_j)$, while moving the complete entries. Ordering <sorting records> by an age field does not also compare their names. Repeated <sort keys> are allowed, so this induces equal-key groups even when the complete <sorting records> differ. Keep the <sort key> <function> and the comparison rule fixed throughout a sort; inconsistent comparisons need not admit any sorted result.
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