(...done...)= items already sorted and (less dups) output
p= the pivot or a duplicate of it
b= "before", things known to be "less than" the pivot
a= "after", things known to be "greater than" the pivot
u= unsorted items (but fit within the current partition)
n= next item to compare to our pivot
(...to-do...)= the partitions to sort after we output this one

(...done...) b b b b b u u u u u n p a a a a a p p p (...to-do...)
if n is a  "b", swap:  b<------->u
if n is an "a", swap:            p-a
if n is a  "p", swap:            p a<------->p