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
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