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This paper is in two parts. The first part deals with the representation of random bilateral transactions in the form of matrices which are, first of all, feasible (Kirchhoff's law) and of which some are secure (Ohm's law). The ratio of secure to feasible is the index POST and is a measure of the transmission system adequacy. It is interesting to note that as the adequacy of the generation system increases, the adequacy of the transmission system decreases simply because there is greater scope for these bilateral transactions to be entered into. The second part of this paper deals with the application of FACTS controllers to improve the security of one or many transactions. It is generally acknowledged that a sufficient number of these controllers can achieve the objective of 100 percent adequacy or close to it. The question is at what cost. It may well turn out that an adequacy of 90 percent is all that can be justified. In any case, it is now evident that the transmission system has not been designed to accommodate unrestricted bilateral trading and that some form of external intervention is required. This paper proposes security indices that could support and justify such intervention.