If a n:rectangle R has a n:length L1 and R has a n:length L2 then R v:has-surface S, and S = ( L1 * L2 ).
Ontology(
   http://attempto.ifi.uzh.ch/ontologies/owlswrl/test
   Implies(
      Antecedent(
         description(
            Class(:rectangle)
            I-variable(1)
         )
         description(
            Class(:length)
            I-variable(2)
         )
         description(
            ObjectProperty(:have)
            I-variable(1)
            I-variable(2)
         )
         description(
            Class(:length)
            I-variable(4)
         )
         description(
            ObjectProperty(:have)
            I-variable(1)
            I-variable(4)
         )
      )
      Consequent(
         description(
            Class(owl:Thing)
            I-variable(6)
         )
         description(
            ObjectProperty(:has-surface)
            I-variable(1)
            I-variable(6)
         )
         builtIn(equal, I-variable(6), I-variable(g1))
         builtIn(multiply, I-variable(g1), I-variable(2), I-variable(4))
      )
   )
)

<?xml version="1.0" encoding="UTF-8"?>

<rdf:RDF
    xml:base="http://attempto.ifi.uzh.ch/ontologies/owlswrl/test"
    xmlns="http://attempto.ifi.uzh.ch/ontologies/owlswrl/test#"
    xmlns:owl="http://www.w3.org/2002/07/owl#"
    xmlns:owl11="http://www.w3.org/2006/12/owl11#"
    xmlns:swrl="http://www.w3.org/2003/11/swrl#"
    xmlns:swrlb="http://www.w3.org/2003/11/swrlb#"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
  <owl:Ontology rdf:about=""/>
  <swrl:Imp>
    <swrl:body>
      <swrl:AtomList>
	<rdf:first>
	  <swrl:ClassAtom>
	    <swrl:argument1 rdf:resource="#x1"/>
	    <swrl:classPredicate rdf:resource="#rectangle"/>
	  </swrl:ClassAtom>
	</rdf:first>
	<rdf:rest>
	  <swrl:AtomList>
	    <rdf:first>
	      <swrl:ClassAtom>
		<swrl:argument1 rdf:resource="#x2"/>
		<swrl:classPredicate rdf:resource="#length"/>
	      </swrl:ClassAtom>
	    </rdf:first>
	    <rdf:rest>
	      <swrl:AtomList>
		<rdf:first>
		  <swrl:IndividualPropertyAtom>
		    <swrl:argument1 rdf:resource="#x1"/>
		    <swrl:argument2 rdf:resource="#x2"/>
		    <swrl:propertyPredicate rdf:resource="#have"/>
		  </swrl:IndividualPropertyAtom>
		</rdf:first>
		<rdf:rest>
		  <swrl:AtomList>
		    <rdf:first>
		      <swrl:ClassAtom>
			<swrl:argument1 rdf:resource="#x4"/>
			<swrl:classPredicate rdf:resource="#length"/>
		      </swrl:ClassAtom>
		    </rdf:first>
		    <rdf:rest>
		      <swrl:AtomList>
			<rdf:first>
			  <swrl:IndividualPropertyAtom>
			    <swrl:argument1 rdf:resource="#x1"/>
			    <swrl:argument2 rdf:resource="#x4"/>
			    <swrl:propertyPredicate rdf:resource="#have"/>
			  </swrl:IndividualPropertyAtom>
			</rdf:first>
			<rdf:rest
			    rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#nil"/>
		      </swrl:AtomList>
		    </rdf:rest>
		  </swrl:AtomList>
		</rdf:rest>
	      </swrl:AtomList>
	    </rdf:rest>
	  </swrl:AtomList>
	</rdf:rest>
      </swrl:AtomList>
    </swrl:body>
    <swrl:head>
      <swrl:AtomList>
	<rdf:first>
	  <swrl:ClassAtom>
	    <swrl:argument1 rdf:resource="#x6"/>
	    <swrl:classPredicate rdf:resource="http://www.w3.org/2002/07/owl#Thing"/>
	  </swrl:ClassAtom>
	</rdf:first>
	<rdf:rest>
	  <swrl:AtomList>
	    <rdf:first>
	      <swrl:IndividualPropertyAtom>
		<swrl:argument1 rdf:resource="#x1"/>
		<swrl:argument2 rdf:resource="#x6"/>
		<swrl:propertyPredicate rdf:resource="#has-surface"/>
	      </swrl:IndividualPropertyAtom>
	    </rdf:first>
	    <rdf:rest>
	      <swrl:AtomList>
		<rdf:first>
		  <swrl:BuiltinAtom>
		    <swrl:builtin rdf:resource="http://www.w3.org/2003/11/swrlb#equal"/>
		    <swrl:arguments>
		      <rdf:Description>
			<rdf:first rdf:resource="#x6"/>
			<rdf:rest>
			  <rdf:Description>
			    <rdf:first rdf:resource="#xg1"/>
			    <rdf:rest
				rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#nil"/>
			  </rdf:Description>
			</rdf:rest>
		      </rdf:Description>
		    </swrl:arguments>
		  </swrl:BuiltinAtom>
		</rdf:first>
		<rdf:rest>
		  <swrl:AtomList>
		    <rdf:first>
		      <swrl:BuiltinAtom>
			<swrl:builtin rdf:resource="http://www.w3.org/2003/11/swrlb#multiply"/>
			<swrl:arguments>
			  <rdf:Description>
			    <rdf:first rdf:resource="#xg1"/>
			    <rdf:rest>
			      <rdf:Description>
				<rdf:first rdf:resource="#x2"/>
				<rdf:rest>
				  <rdf:Description>
				    <rdf:first rdf:resource="#x4"/>
				    <rdf:rest
					rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#nil"/>
				  </rdf:Description>
				</rdf:rest>
			      </rdf:Description>
			    </rdf:rest>
			  </rdf:Description>
			</swrl:arguments>
		      </swrl:BuiltinAtom>
		    </rdf:first>
		    <rdf:rest
			rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#nil"/>
		  </swrl:AtomList>
		</rdf:rest>
	      </swrl:AtomList>
	    </rdf:rest>
	  </swrl:AtomList>
	</rdf:rest>
      </swrl:AtomList>
    </swrl:head>
  </swrl:Imp>
  <rdf:Description rdf:about="#rectangle">
    <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#Class"/>
  </rdf:Description>
  <rdf:Description rdf:about="#x1">
    <rdf:type rdf:resource="http://www.w3.org/2003/11/swrl#Variable"/>
  </rdf:Description>
  <rdf:Description rdf:about="#length">
    <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#Class"/>
  </rdf:Description>
  <rdf:Description rdf:about="#x2">
    <rdf:type rdf:resource="http://www.w3.org/2003/11/swrl#Variable"/>
  </rdf:Description>
  <rdf:Description rdf:about="#have">
    <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#ObjectProperty"/>
  </rdf:Description>
  <rdf:Description rdf:about="#x4">
    <rdf:type rdf:resource="http://www.w3.org/2003/11/swrl#Variable"/>
  </rdf:Description>
  <rdf:Description rdf:about="http://www.w3.org/2002/07/owl#Thing">
    <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#Class"/>
  </rdf:Description>
  <rdf:Description rdf:about="#x6">
    <rdf:type rdf:resource="http://www.w3.org/2003/11/swrl#Variable"/>
  </rdf:Description>
  <rdf:Description rdf:about="#has-surface">
    <rdf:type rdf:resource="http://www.w3.org/2002/07/owl#ObjectProperty"/>
  </rdf:Description>
  <rdf:Description rdf:about="#xg1">
    <rdf:type rdf:resource="http://www.w3.org/2003/11/swrl#Variable"/>
  </rdf:Description>
</rdf:RDF>
If a rectangle R has a length L1 and R has a length L2 then R v:has-surface ( L1 * L2 ).
Ontology(
   http://attempto.ifi.uzh.ch/ontologies/owlswrl/test
   Implies(
      Antecedent(
         description(
            Class(:rectangle)
            I-variable(1)
         )
         description(
            Class(:length)
            I-variable(2)
         )
         description(
            ObjectProperty(:have)
            I-variable(1)
            I-variable(2)
         )
         description(
            Class(:length)
            I-variable(4)
         )
         description(
            ObjectProperty(:have)
            I-variable(1)
            I-variable(4)
         )
      )
      Consequent(
         description(
            DataProperty(:has-surface)
            I-variable(1)
            I-variable(g1)
         )
         builtIn(multiply, I-variable(g1), I-variable(2), I-variable(4))
      )
   )
)

ERROR: Unexpected error.
If there is a circle C and C's radius is R and C's area is S then S = Pi * (R * R) / 1.
Ontology(
   http://attempto.ifi.uzh.ch/ontologies/owlswrl/test
   Implies(
      Antecedent(
         description(
            ObjectProperty(:area)
            I-variable(2)
            I-variable(3)
         )
         description(
            ObjectProperty(:radius)
            I-variable(2)
            I-variable(5)
         )
         description(
            Class(:circle)
            I-variable(2)
         )
         description(
            Class(owl:Thing)
            I-variable(5)
         )
         description(
            Class(owl:Thing)
            I-variable(3)
         )
      )
      Consequent(
         builtIn(equal, I-variable(3), I-variable(g1))
         builtIn(divide, I-variable(g1), I-variable(g2), ^^(1, http://www.w3.org/2001/XMLSchema#integer))
         builtIn(multiply, I-variable(g2), Individual(:Pi), I-variable(g3))
         builtIn(multiply, I-variable(g3), I-variable(5), I-variable(5))
      )
   )
)

ERROR: Unexpected error.