Guessing the Type of a Newly Admitted Function

When a function is admitted to the logic, ACL2 tries to ``guess'' what type of object it returns. This guess is codified as a term that expresses a property of the value of the function. For app the term is

    (EQUAL (APP X Y) Y))
which says that app returns either a cons or its second argument. This formula is added to ACL2's rule base as a type-prescription rule. Later we will discuss how rules are used by the ACL2 theorem prover. The point here is just that when you add a definition, the data base of rules is updated, not just by the addition of the definitional axiom, but by several new rules.

You should now return to the Walking Tour.