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There are 13 hex-Rings (A) and 8 pent-Rings (B) that may be grouped by number and arrangement of DADs. I: Rings with no DAD edges. II: Rings with two DAD edges. III: Rings with four DAD edges, arranged head-to-head or tail-to-tail. IV: Rings with four DAD edges, with one or two head-to-tail arrangements of DADs. The latter Rings are marked with asterisks.
