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2.7.11 Predicates

A couple of operations are built into the virtual machine for performing tests efficiently. These functions return either nil for false or (nil,nil) for true, and are useful for example as a predicate p in programs of the form conditional(p,(f,g)) among other things. In this example, the predicate is applied to the argument, a result of (nil,nil) causes f to be applied to it, and a result of nil causes g to be applied to it.


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