YES

certificate:
<?xml version='1.0' ?><certificationProblem xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="cpf3.xsd"><cpfVersion>3.0</cpfVersion><lookupTables /><input><infeasibilityInput><ctrs><conditionType><oriented /></conditionType><rules><rule><funapp><name>seven</name></funapp><funapp><name>s</name><funapp><name>s</name><funapp><name>s</name><funapp><name>s</name><funapp><name>s</name><funapp><name>s</name><funapp><name>s</name><funapp><name>O</name></funapp></funapp></funapp></funapp></funapp></funapp></funapp></funapp><conditions /></rule><rule><funapp><name>isNat</name><funapp><name>false</name></funapp></funapp><funapp><name>false</name></funapp><conditions /></rule><rule><funapp><name>isNat</name><funapp><name>true</name></funapp></funapp><funapp><name>false</name></funapp><conditions /></rule><rule><funapp><name>isNat</name><funapp><name>s</name><var>m</var></funapp></funapp><funapp><name>isNat</name><var>m</var></funapp><conditions /></rule><rule><funapp><name>isNat</name><funapp><name>O</name></funapp></funapp><funapp><name>true</name></funapp><conditions /></rule><rule><funapp><name>ack</name><funapp><name>O</name></funapp><var>n</var></funapp><funapp><name>s</name><var>n</var></funapp><conditions /></rule><rule><funapp><name>ack</name><funapp><name>s</name><var>m</var></funapp><funapp><name>O</name></funapp></funapp><funapp><name>ack</name><var>m</var><funapp><name>s</name><funapp><name>O</name></funapp></funapp></funapp><conditions /></rule><rule><funapp><name>ack</name><funapp><name>s</name><var>m</var></funapp><funapp><name>s</name><var>n</var></funapp></funapp><funapp><name>ack</name><var>m</var><funapp><name>ack</name><funapp><name>s</name><var>m</var></funapp><var>n</var></funapp></funapp><conditions /></rule></rules></ctrs><infeasibilityQuery><rule><funapp><name>isNat</name><funapp><name>ack</name><funapp><name>seven</name></funapp><funapp><name>seven</name></funapp></funapp></funapp><funapp><name>false</name></funapp></rule></infeasibilityQuery></infeasibilityInput></input><property><infeasibility /></property><answer><yes /></answer><proof><infeasibilityProof><infeasibleEquation><rule><funapp><name>isNat</name><funapp><name>ack</name><funapp><name>seven</name></funapp><funapp><name>seven</name></funapp></funapp></funapp><funapp><name>false</name></funapp></rule><nonreachabilityProof><nonreachableCorewritePair><weightedPathOrder><precedenceStatus><precedenceStatusEntry><name>seven</name><arity>0</arity><precedence>0</precedence><status /></precedenceStatusEntry><precedenceStatusEntry><name>s</name><arity>1</arity><precedence>1</precedence><status /></precedenceStatusEntry><precedenceStatusEntry><name>O</name><arity>0</arity><precedence>5</precedence><status /></precedenceStatusEntry><precedenceStatusEntry><name>isNat</name><arity>1</arity><precedence>2</precedence><status /></precedenceStatusEntry><precedenceStatusEntry><name>false</name><arity>0</arity><precedence>6</precedence><status /></precedenceStatusEntry><precedenceStatusEntry><name>true</name><arity>0</arity><precedence>3</precedence><status /></precedenceStatusEntry><precedenceStatusEntry><name>ack</name><arity>2</arity><precedence>4</precedence><status /></precedenceStatusEntry></precedenceStatus><interpretation><type><polynomial><domain><naturals /></domain><degree>1</degree></polynomial></type><interpret><name>seven</name><arity>0</arity><integer>2</integer></interpret><interpret><name>s</name><arity>1</arity><sum><integer>0</integer><product><integer>1</integer><variable>1</variable></product></sum></interpret><interpret><name>O</name><arity>0</arity><integer>1</integer></interpret><interpret><name>isNat</name><arity>1</arity><sum><integer>4</integer><product><integer>1</integer><variable>1</variable></product></sum></interpret><interpret><name>false</name><arity>0</arity><integer>7</integer></interpret><interpret><name>true</name><arity>0</arity><integer>4</integer></interpret><interpret><name>ack</name><arity>2</arity><sum><integer>0</integer><product><integer>0</integer><variable>1</variable></product><product><integer>1</integer><variable>2</variable></product></sum></interpret></interpretation></weightedPathOrder></nonreachableCorewritePair></nonreachabilityProof></infeasibleEquation></infeasibilityProof></proof></certificationProblem>