: partition ( L R -- L1 R1 L2 R2 )
    2dup  dup @ >r
    begin 2dup < while
	    begin dup @ r@  > while  cell- repeat swap
	    begin r@ over @ > while  cell+ repeat swap
	    2dup <= if 2dup exchange  cell- swap cell+ swap  then
    repeat
    r> drop    ( L R L2 R1 )
    swap rot   ( L R1 L2 R )
;