(VAR x y z)
(RULES 
qsort(nil) -> nil
qsort(.(x,y)) -> ++(qsort(lowers(x,y)),.(x,qsort(greaters(x,y))))
lowers(x,nil) -> nil
lowers(x,.(y,z)) -> if(<=(y,x),.(y,lowers(x,z)),lowers(x,z))
greaters(x,nil) -> nil
greaters(x,.(y,z)) -> if(<=(y,x),greaters(x,z),.(y,greaters(x,z)))
)
(COMMENT Example 4.59 (Quick-Sort) in \cite{SK90})