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.