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A transition system whose edges carry action labels. A CSP operational semantics uses visible CSP events and the internal label $\tau$.

Write $J_A(P,Q)=P{△}_{A}Q$. The following five CSP operational semantics rules suffice, with $a$ always visible:Thus a CSP internal action of either operand preserves the wrapper. A visible action outside $A$ by the left operand also preserves it, whereas such an action by the right operand performs an ordinary CSP interrupt and discards the left operand. An action in $A$ requires both operands and advances both without discarding either. If both can perform the same event outside $A$, there are two transitions with the same visible label and potentially different derivatives; neither should be deleted. The paper excludes successful termination, so no extra termination rules are needed.