## 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

(OR (CONSP (APP X Y))
(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.