8. The system of claim 7 , wherein each first substitution context includes an additional context term that is absent from the second substitution context.
9. The system of claim 8 , wherein the operations further comprise: generating a context hierarchy from the plurality of substitution contexts for the original term, wherein any context terms of a parent substitution context in the context hierarchy also occur in each of one or more child substitution contexts of the parent substitution context, and wherein each child substitution context in the context hierarchy includes all of the context terms of the parent substitution context of the child substitution context as well as the additional context term that is absent from the parent substitution context of the child substitution context; and selecting each different second substitution context as a parent of a parent-child relationship with the first substitution context in the context hierarchy.