lower town        upper town

    a -> b	    	A -> B
    b -> c	    	B -> C
    c -> d      
    d -> e  	        D -> E
    e -> f	        E -> F

##</code><code>##

A,D,B,C,E,F
D,E,F,A,B,C

(AD),(BE),(CF) # with a parallel machine