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A Formally Verified HOL4 Algebra for Event Trees

Event Tree (ET) analysis is widely used as a forward deductive safety analysis technique for decision-making at the critical-system design stage. ET is a schematic diagram representing all possible operating states and external events in a system so that one of these possible scenarios can occur. In this report, we propose to use the HOL4 theorem prover for the formal modeling and step-analysis of ET diagrams. To this end, we developed a formalization of ETs in higher-order logic, which is based on a generic list datatype that can: (i) construct an arbitrary level of ET diagrams; (ii) reduce the irrelevant ET branches; (iii) partition ET paths; and (iv) perform the probabilistic analysis based on the occurrence of certain events. For illustration purposes, we conduct the formal ET stepwise analysis of an electrical power grid and also determine its System Average Interruption Frequency Index (SAIFI), which is an important indicator for system reliability.