If it is only required to know how many horizontal strips of a given
length there are, but not their exact composition, option 8 may
be used. The de Bruijn matrix and its first eight powers are
calculated; their 
elements and traces are tabulated.
The 
elements tell how many isolated strips there are, the trace
the number of periodic strips. Not all these strips will stack
vertically, so the numbers represent an upper bound (but often a fair
estimate) for the number of nodes in the second level diagram. The
maximum number of links each node may have is 
(isolated) or
(periodic), for length l; in practice the number is much, much
lower.