Mutation testing researchers have long known that many generated mutants are not needed. This paper develops a graph model to describe redundancy among mutations. We define "true" subsumption, a relation that practicing test engineers would like to have, but cannot due to issues of computability. We also define dynamic subsumption and static subsumption as approximations of "true" subsumption. We explore the properties of the approximate subsumption relations in the context of a small example. We suggest possible uses for subsumption graphs.
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