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<head><title>ACL2-PC_colon__colon_PUT.html -- ACL2 Version 3.1</title></head>
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<h3>ACL2-PC::PUT</h3>(macro)
<code> </code>substitute for a ``free variable''
<pre>Major Section: <a href="PROOF-CHECKER-COMMANDS.html">PROOF-CHECKER-COMMANDS</a>
</pre><p>
<pre>
Example:
(put x 17)
<p>
General Form:
(put var expr)
</pre>
Substitute <code>expr</code> for the ``free variable'' <code>var</code>, as explained below.<p>
A ``free variable'' is, for our purposes, a variable <code>var</code> such that
the instruction <code>(free var)</code> has been executed earlier in the
state-stack. What <code>(free var)</code> really does is to let <code>var</code> be an
abbreviation for the term <code>(hide var)</code> (see documentation for
<code>add-abbreviation</code>). What <code>(put var expr)</code> really does is to unwind the
state-stack, replacing that <code>free</code> instruction with the instruction
<code>(add-abbreviation var expr)</code>, so that future references to <code>(? var)</code>
become reference to <code>expr</code> rather than to <code>(hide var)</code>, and then to
replay all the other instructions that were unwound. Because <code>hide</code>
was used, the expectation is that in most cases, the instructions
will replay successfully and <code>put</code> will ``succeed''. However, if any
replayed instruction ``fails'', then the entire replay will abort
and ``fail'', and the state-stack will revert to its value before
the <code>put</code> instruction was executed.<p>
If <code>(put var expr)</code> ``succeeds'', then <code>(remove-abbreviation var)</code> will
be executed at the end.<p>
<strong>Note</strong>: The <code>restore</code> command will revert the state-stack to its value
present before the <code>put</code> instruction was executed.
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