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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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