Major Section: EVENTS

Examples: (defchoose choose-x-for-p1-and-p2 (x) (y z) (and (p1 x y z) (p2 x y z)))(defchoose choose-x-for-p1-and-p2 x (y z) ; equivalent to the above (and (p1 x y z) (p2 x y z)))

; The following is as above, but strengthens the axiom added to pick a sort ; of canonical witness, as described below. (defchoose choose-x-for-p1-and-p2 x (y z) (and (p1 x y z) (p2 x y z)) :strengthen t)

(defchoose choose-x-and-y-for-p1-and-p2 (x y) (z) (and (p1 x y z) (p2 x y z)))

## Some Related Topics

### CONSERVATIVITY-OF-DEFCHOOSE -- proof of conservativity of

`defchoose`

`fn`

is the symbol you wish to define and is a new symbolic
name (see name), `(bound-var1 ... bound-varn)`

is a list of
distinct `bound' variables (see below), `(free-var1 ... free-vark)`

is the list of formal parameters of `fn`

and is disjoint from the
bound variables, and `body`

is a term. The use of `lambda-list`

keywords (such as `&optional`

) is not allowed. The documentation
string argument, `:doc doc-string`

, is optional; for a description of the
form of `doc-string`

see doc-string. The `:strengthen`

keyword argument
is optional; if supplied, it must be `t`

or `nil`

.
The system treats `fn`

very much as though it were declared in the
signature of an `encapsulate`

event, with a single axiom exported as
described below. If you supply a `:use`

hint (see hints), `:use fn`

, it
will refer to that axiom. No rule (of class `:`

`rewrite`

or otherwise;
see rule-classes) is created for `fn`

.

`Defchoose`

is only executed in defun-mode `:`

`logic`

;
see defun-mode. Also see defun-sk.

In the most common case, where there is only one bound variable, it is
permissible to omit the enclosing parentheses on that variable. The effect
is the same whether or not those parentheses are omitted. We describe this
case first, where there is only one bound variable, and then address the
other case. Both cases are discussed assuming `:strengthen`

is `nil`

,
which is the default. We deal with the case `:strengthen t`

at the end.

The effect of the form

(defchoose fn bound-var (free-var1 ... free-vark) body)is to introduce a new function symbol,

`fn`

, with formal parameters
`(free-var1 ... free-vark)`

. Now consider the following axiom, which
states that `fn`

picks a value of `bound-var`

so that the body will be
true, if such a value exists:
(1) (implies body (let ((bound-var (fn free-var1 ... free-vark))) body))This axiom is ``clearly conservative'' under the conditions expressed above: the function

`fn`

simply picks out a ``witnessing'' value of `bound-var`

if there is one. For a rigorous statement and proof of this conservativity
claim, see conservativity-of-defchoose.Next consider the case that there is more than one bound variable, i.e., there is more than one bound-var in the following.

(defchoose fn (bound-var1 ... bound-varn) (free-var1 ... free-vark) body)Then

`fn`

returns a multiple value with `n`

components, and formula (1)
above is expressed using `mv-let`

as follows:
(implies body (mv-let (bound-var1 ... bound-varn) (fn free-var1 ... free-vark) body))

We now discuss the case that `:strengthen t`

is supplied. For simplicity
we return to our simplest case, with `defchoose`

applied to function
`fn`

, a single free variable `y`

, and a single bound variable
`bound-var`

. The idea is that if we pick the ``smallest'' witnessing
`bound-var`

for two different free variables `y`

and `y1`

, then either
those two witnesses are the same, or else one is less than the other, in
which case the smaller one is a witness for its free variable but not for the
other. (See comments in source function `defchoose-constraint-extra`

for
more details.) Below, `body1`

is the result of replacing `y`

by `y1`

in `body`

.

(2) (or (equal (fn y) (fn y1)) (let ((bound-var (fn y))) (and body (not body1))) (let ((bound-var (fn y1))) (and body1 (not body))))An important application of this additional axiom is to be able to define a ``fixing'' function that picks a canonical representative of each equivalence class, for a given equivalence relation. The following events illustrate this point.

(encapsulate ((equiv (x y) t)) (local (defun equiv (x y) (equal x y))) (defequiv equiv))(defchoose efix (x) (y) (equiv x y) :strengthen t)

(defthm equiv-implies-equal-efix-1 (implies (equiv y y1) (equal (efix y) (efix y1))) :hints (("Goal" :use efix)) :rule-classes (:congruence))

(defthm efix-fixes (equiv (efix x) x) :hints (("Goal" :use ((:instance efix (y x))))))

If there is more than one bound variable, then (2) is modified in complete
analogy to (1) to use `mv-let`

in place of `let`

.

Comment for logicians: As we point out in the documentation for
`defun-sk`

, `defchoose`

is ``appropriate,'' by which we mean that
it is conservative, even in the presence of `epsilon-0`

induction.
For a proof, See conservativity-of-defchoose.