In this paper we address the problem of subsumption checking for subscriptions in pub/sub systems. We develop a novel approach based on negative space representation for subsumption checking and provide efficient algorithms for subscription forwarding in a dynamic pub/sub environment. We then provide heuristics for approximate subsumption checking that greatly enhance the performance without compromising the correct execution of the system and only adding incremental cost in terms of extra computation in brokers. We illustrate the advantages of this novel approach by carrying out extensive experimentation.