Photograph of proto-cuneiform tablet showing signs discussed in the article. (Colour image of drawing of Fig. 4 in the article - Ref_ Englund 1994 ATU 5_ pl. 71).

The stacks of rings catalyse the conversion of the basic building blocks into new three- and four-membered rings which are needed for the growth of stacks of six-membered rings (self-replication ...