## Common Lisp as a Modeling Language

In ACL2 we have adopted Common Lisp as the basis of our modeling language.
If you have already read our brief note on Common Lisp and recall
the example of `app`

, please proceed. Otherwise
click here for an exceedingly brief introduction to
Common Lisp and then come **back** here.

In Common Lisp it is very easy to write systems of formulas that
manipulate discrete, inductively constructed data objects. In building
a model you might need to formalize the notion of sequences and define
such operations as concatenation, length, whether one is a permutation
of the other, etc. It is easy to do this in Common Lisp. Furthermore,
if you have a Common Lisp ``theory of sequences'' you can **run**
the operations and relations you define. That is, you can execute
the functions on concrete data to see what results your formulas produce.

If you define the function `app`

as shown above and then type

(app '(A B) '(C D E))

in any Common Lisp, the answer will be computed and will be
`(A B C D E)`

.
The **executable** nature of Common Lisp and thus of ACL2 is very handy
when producing models.

But executability is not enough for a modeling language because the purpose
of models is to permit analysis.

Click here to continue.