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of theory. I will only say that in formulating that rule, I considered that of the six fundamental permissible operations, two are non-reversible, namely erasure and insertion. Where That is where there can either of these can be performed the reverse operation cannot be performed. I further considered that three of the six fundamental permissible operations complicate the graph, while the other three simplify it. Namely, insertion, iteration, and the placing of marking introduction of a double cut, complicate the graph, while erasure, deiteration, and the removal of a double cut simplify it. It is necessary for the purpose to modify the non-reversible operations so as to render them reversible; and it fortunately happens