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Over the last two decades, a number of deadlock control policies based on Petri nets were proposed for flexible manufacturing systems (FMSs). As a structural object of a Petri net, siphons are widely used in deadlock control. For system of sequential systems with shared resources (S4R), the current deadlock control policies based on max or max'-controlled siphons tend to overly restrict the behaviour of a controlled system. The controllability conditions of a siphon are relaxed by a new concept called max'-controlled ones. We conclude that an S4R is live if all its siphons are max"-controlled. Compared with the existing ones, the proposed one is more general. Examples are given to illustrate it.