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<head><title>INVISIBLE-FNS-TABLE.html -- ACL2 Version 3.1</title></head>
<body text=#000000 bgcolor="#FFFFFF">
<h2>INVISIBLE-FNS-TABLE</h2>functions that are invisible to the <a href="LOOP-STOPPER.html">loop-stopper</a> algorithm
<pre>Major Section: <a href="EVENTS.html">EVENTS</a>
</pre><p>
<pre>
Examples:
ACL2 !>(invisible-fns-table (w state))
((binary-+ unary--)
(binary-* unary-/)
(unary-- unary--)
(unary-/ unary-/))
</pre>
Among other things, the setting above has the effect of making <code><a href="UNARY--.html">unary--</a></code>
``invisible'' for the purposes of applying permutative <code>:</code><code><a href="REWRITE.html">rewrite</a></code>
rules to <code><a href="BINARY-+.html">binary-+</a></code> trees. Also see <a href="ADD-INVISIBLE-FNS.html">add-invisible-fns</a>,
see <a href="REMOVE-INVISIBLE-FNS.html">remove-invisible-fns</a>, and see <a href="SET-INVISIBLE-FNS-TABLE.html">set-invisible-fns-table</a>.<p>
See <a href="TABLE.html">table</a> for a general discussion of tables.
<p>
The ``invisible functions <a href="TABLE.html">table</a>'' is an alist with elements of the following
form, where <code>fn</code> is a function symbol and the <code>ufni</code> are unary function
symbols in the current ACL2 <a href="WORLD.html">world</a>, and <code>k</code> is at least 1.
<pre>
(fn ufn1 ufn2 ... ufnk)
</pre>
<p>
This <a href="TABLE.html">table</a> thus associates with certain function symbols, e.g., <code>fn</code>
above, a set of unary functions, e.g., the <code>ufni</code> above. The <code>ufni</code>
associated with <code>fn</code> in the invisible functions table are said to be
``invisible with respect to <code>fn</code>.'' If <code>fn</code> is not the <code><a href="CAR.html">car</a></code> of any
pair in the <code>alist</code>, then no function is invisible for it. Thus for
example, setting the invisible functions alist to <code>nil</code> completely
eliminates the consideration of invisibility.<p>
The notion of invisibility is involved in the use of the
<code>:</code><code><a href="LOOP-STOPPER.html">loop-stopper</a></code> field of <code>:</code><code><a href="REWRITE.html">rewrite</a></code> rules to prevent the indefinite
application of permutative rewrite rules. Roughly speaking, if
rewrite rules are being used to permute <code>arg</code> and (ufni arg) inside of
a nest of <code>fn</code> calls, and <code>ufni</code> is invisible with respect to <code>fn</code>, then
<code>arg</code> and <code>(ufni arg)</code> are considered to have the same ``weight'' and
will be permuted so as to end up as adjacent tips in the <code>fn</code> nest.
See <a href="LOOP-STOPPER.html">loop-stopper</a>.
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