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<p>The following table compares key features of several theorem-provers and proof-checking systems. 
You will note that Otter-&lambda; is the only one without a "no" entry in any column. The 
first column, labeled <em>Higher Order Unification</em>, refers to the use of higher-order or second-order 
unification, or in Otter-&lambda;'s case, untyped lambda-unification. The entry for Otter-&lambda; is "some" rather 
than "yes", since Otter-&lambda; at present returns only one unifier, when there may be several; but on the other hand, 
Otter-&lambda; can succeed in some untyped unifications that fail in typed systems. The second 
column <em>Accumulates Conclusion</em> refers to the ability to accumulate derived conclusions 
in a "database" of deduced conclusions, that can be searched for the generation of new conclusions. 
The column <em>Proof Search</em> refers to the ability to search for a proof (as opposed to checking a proof 
entered by the user or developed interactively). The last column refers to whether the system can allow the user 
precise control over the rules of inference to be used, and to specify "strategies" that control the proof search, 
such as discarding or favoring formulas of certain forms. </p>
<p>&nbsp;</p>
<table width="83%" height="246" border="1" align="center" cellpadding="0" cellspacing="0" bordercolor="#FFFFFF" bgcolor="#ECE9D8" id="AutoNumber1" style="border-collapse: collapse">
      <tr>
        <td width="20%">
        <p align="center"><strong>System</strong></td>
        <td width="19%">
        <p align="center"><strong>Higher Order Unification</strong></td>
        <td width="18%">
        <p align="center"><strong>Accumulates Conclusions</strong></td>
        <td width="21%">
        <p align="center"><strong>Proof Search</strong></td>
        <td width="22%">
        <p align="center"><strong>Can vary rules and strategies</strong></td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">PVS</td>
        <td width="19%">
        <p align="center">Matching</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">Some</td>
        <td width="22%">
        <p align="center">Yes</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">HOL-Light</td>
        <td width="19%">
        <p align="center">Yes</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">MESON_TAC</td>
        <td width="22%">
        <p align="center">No</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">Isabelle</td>
        <td width="19%">
        <p align="center">Yes</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">No</td>
        <td width="22%">
        <p align="center">Yes</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">NuPrl</td>
        <td width="19%">
        <p align="center">No</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">No</td>
        <td width="22%">
        <p align="center">No</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">Coq</td>
        <td width="19%">
        <p align="center">Yes</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">No</td>
        <td width="22%">
        <p align="center">No</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">Theorema</td>
        <td width="19%">
        <p align="center">No</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">Some</td>
        <td width="22%">
        <p align="center">Yes</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">&lambda;-Prolog</td>
        <td width="19%">
        <p align="center">Yes</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">HH formulas only</td>
        <td width="22%">
        <p align="center">No</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">TPS</td>
        <td width="19%">
        <p align="center">Yes</td>
        <td width="18%">
        <p align="center">Yes</td>
        <td width="21%">
        <p align="center">Yes</td>
        <td width="22%">
        <p align="center">Some</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center">ACL2</td>
        <td width="19%">
        <p align="center">No</td>
        <td width="18%">
        <p align="center">No</td>
        <td width="21%">
        <p align="center">Yes</td>
        <td width="22%">
        <p align="center">No</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center"><a href="http://www-unix.mcs.anl.gov/AR/otter/">Otter</a></td>
        <td width="19%">
        <p align="center">No</td>
        <td width="18%">
        <p align="center">Yes</td>
        <td width="21%">
        <p align="center">Yes</td>
        <td width="22%">
        <p align="center">Yes</td>
      </tr>
      <tr>
        <td width="20%">
        <p align="center"><a href="http://mh215a.cs.sjsu.edu/index.php/">Otter-&lambda;</a></td>
        <td width="19%">
        <p align="center">Some</td>
        <td width="18%">
        <p align="center">Yes</td>
        <td width="21%">
        <p align="center">Yes</td>
        <td width="22%">
        <p align="center">Yes</td>
      </tr>
</table>
<p>&nbsp;</p>