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// Quick sort
// Rules
qsort(ret) >< [] => ret~[];
qsort(ret) >< (int x):xs =>
Append(ret, x:right)~left, part(smaller, larger, x)~xs,
qsort(left)~smaller, qsort(right)~larger;
// Note: `Append' is implemented as the following built-in agent:
// Append(ret, b)~a --> ret ~ (a++b)
part(smaller, larger, int x) >< [] => smaller~[], larger~[];
part(smaller, larger, int x) >< (int y):ys
| y<x => smaller~(y:cnt), part(cnt, larger, x)~ys
| _ => larger~(y:cnt), part(smaller, cnt, x)~ys;
// Nets
qsort(ret)~[3,6,1,9,2];
ret;
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