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# 28 - 1 Program modules can import other modules[You have...

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1. Program modules can import other modules. [You have seen this al- ready in the Haskell course.] They can also depend indirectly on mod- ules via some chain of importation, so that for instance M depends on M if M imports M and M imports M . The relation ‘depends’ is the transitive closure of the relation ‘imports’. 2. Two people are related if one is the parent of the other, if they are mar- ried, or if there is a chain of such relationships joining them directly. We can model this by a universal set People , with three relations Mar- ried , Parent and Relative . The relation Relative is defined using the transitive closure: Relative = (( Parent Parent - 1 ) Married ) + D EFINITION 3.16 (T RANSITIVE C LOSURE ) Let R be a binary relation on A . The transitive closure of R , written t ( R ) or R + , is n 1 R n : that is, R R 2 R 3 R 4 . . . The transitive closure of a binary relation always exists. To cast more light on R + , we now examine an alternative way of building the transitive clo- sure. Let R
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