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<head><title>UBT.html -- ACL2 Version 3.1</title></head>
<body text=#000000 bgcolor="#FFFFFF">
<h2>UBT</h2>undo the <a href="COMMAND.html">command</a>s back through a <a href="COMMAND.html">command</a> descriptor
<pre>Major Section: <a href="HISTORY.html">HISTORY</a>
</pre><p>
<pre>
Examples:
:ubt :max ; undo back through the most recent command
; (which just means undo the most recent command)
:ubt :x ; same as :ubt :max
:u ; same as :ubt :max with no questions asked
:ubt fn ; undo back through the introduction of fn
; (including all the other events in fn's block)
:ubt 5 ; undo back through the fifth command executed
:ubt (:max -4) ; undo back through the most recent five commands
:ubt (:x -4) ; undo back through the most recent five commands
</pre>
See <a href="COMMAND-DESCRIPTOR.html">command-descriptor</a>.
<p>
<code>Ubt</code> takes one argument, a <a href="COMMAND.html">command</a> descriptor, and undoes the
<a href="COMMAND.html">command</a>s from <code>:</code><code><a href="MAX.html">max</a></code> (aka <code>:x</code>) through the one described.
See <a href="COMMAND-DESCRIPTOR.html">command-descriptor</a>. <code><a href="PBT.html">Pbt</a></code> will print the <a href="COMMAND.html">command</a>s that <code>ubt</code>
will undo. <code>:</code><code><a href="OOPS.html">Oops</a></code> will undo the undo. See <a href="OOPS.html">oops</a>.<p>
<code>Ubt</code> can cause errors or queries. To avoid these, see <a href="UBT_bang_.html">ubt!</a>.<p>
It is important to remember that a <a href="COMMAND.html">command</a> may create several
<a href="EVENTS.html">events</a>. That is, the <a href="COMMAND.html">command</a> that introduces <code>fn1</code> may also introduce
<code>fn2</code>. Undoing the <a href="COMMAND.html">command</a> that created either of these will undo
them both. The <a href="EVENTS.html">events</a> created by a <a href="COMMAND.html">command</a> constitute the <a href="COMMAND.html">command</a>'s
``block'' and we can only undo entire blocks. Use <code><a href="PCB.html">pcb</a></code> to print the
<a href="COMMAND.html">command</a> block of a <a href="COMMAND.html">command</a> if you wish to see what will be lost by
undoing the <a href="COMMAND.html">command</a>.<p>
<code>Ubt</code> will not undo into ``prehistory''. <code>:Ubt 1</code> will undo all of your
<a href="COMMAND.html">command</a>s. But <code>:ubt -5</code> will cause an error, warning you that <code>:ubt</code>
cannot undo system initialization.<p>
See <a href="U.html">u</a> for how to undo just the latest command, and see <a href="UBU.html">ubu</a> and see <a href="UBU_bang_.html">ubu!</a> for
how to undo back up to, but not including, the current command.
<br><br><br><a href="acl2-doc.html"><img src="llogo.gif"></a> <a href="acl2-doc-index.html"><img src="index.gif"></a>
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