MAYBE
<?xml version="1.0"?>
<?xml-stylesheet type="text/xsl" href="cpfHTML.xsl"?>
<certificationProblem xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../xml/cpf.xsd">
<input>
 <trsInput>
  <trs>
   <rules>
    <rule>
     <lhs>
      <funapp>
       <name>f</name>
       <arg>
        <funapp>
         <name>a</name>
        </funapp>
       </arg>
       <arg>
        <funapp>
         <name>f</name>
         <arg>
          <funapp>
           <name>a</name>
          </funapp>
         </arg>
         <arg>
          <var>x</var>
         </arg>
        </funapp>
       </arg>
      </funapp>
     </lhs>
     <rhs>
      <funapp>
       <name>b</name>
      </funapp>
     </rhs>
    </rule>
   </rules>
  </trs>
 </trsInput>
</input>
<cpfVersion>2.1</cpfVersion>
<origin>
 <proofOrigin>
  <tool>
   <name>ACP</name>
   <version>0.72</version>
  </tool>
 </proofOrigin>
</origin>
</certificationProblem>