Alerted by a post of sigfpe, I learnt about George Spencer-Brown’s 1972 book Laws of Form.  Reading Louis Kauffman’s account of the theory, I was struck by the similarity to Robin Milner‘s theory of bigraphs (see here for papers). From a talk I heard him give a few years ago, I believe that Milner’s theory was originally intended as a rigorous category-theoretic account of hyperlinks in computer networks.  Has anyone explored the connections between these two mathematical theories?