<?xml version="1.0" encoding="UTF-8"?>

<Rulebase xmlns="http://www.ruleml.org/0.87/xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.ruleml.org/0.87/xsd http://www.ruleml.org/0.87/xsd/datalog.xsd">

<!-- start XML comment ...

Note: This is an expanded (i.e. maximally-roled) version of this example.

This example rulebase contains four rules.
The first and second rules are implications; the third and fourth ones are facts.

In English:

The first rule implies that a person owns an object
if that person buys the object from a merchant and the person keeps the object.

As an OrdLab Tree:

 imp~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          *                         *
     head *                    body *
          *                         *
        atom~~~~~~~~~~~~~~~~~~     and~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                 *     |     |           |                                   |
             opr *     |     |           |                                   |
                 *     |     |           |                                   |        
                rel   var   var        atom~~~~~~~~~~~~~~~~~~~~~~~~~~~     atom~~~~~~~~~~~~~~~~~~
                 .     .     .                  *     |      |       |              *     |     |
                 .     .     .              opr *     |      |       |          opr *     |     |
                 .     .     .                  *     |      |       |              *     |     |
                own  person object             rel   var    var     var            rel   var   var
                                                .     .      .       .              .     .     . 
                                                .     .      .       .              .     .     .
                                                .     .      .       .              .     .     .
                                               buy  person merchant object        keep  person object

... end XML comment -->


<clause>
	<Imp>
	  <head>
	    <Atom>
	      <opr><Rel>own</Rel></opr>
	      <arg index="1"><Var>person</Var></arg>
	      <arg index="2"><Var>object</Var></arg>
	    </Atom>
	  </head>
	  <body>
	    <!-- explicit 'and' -->
	    <And>
	      <Atom>
	        <opr><Rel>buy</Rel></opr>
	        <arg index="3"><Var>object</Var></arg>
	        <arg index="2"><Var>merchant</Var></arg>
      	        <arg index="1"><Var>person</Var></arg>
	      </Atom>
	      <Atom>
	        <opr><Rel>keep</Rel></opr>
	        <arg index="2"><Var>object</Var></arg>
	        <arg index="1"><Var>person</Var></arg>
	      </Atom>
	    </And>
	  </body>
	</Imp>
  </clause>

<!-- The second rule implies that a person buys an object from a merchant
if the merchant sells the object to the person. -->

<clause>
	<Imp>
	  <head>
	    <Atom>
	      <opr><Rel>buy</Rel></opr>
	      <arg index="2"> <Var>merchant</Var></arg>
	      <arg index="1"><Var>person</Var></arg>
	      <arg index="3"><Var>object</Var></arg>
	    </Atom>
	  </head>
	  <body>
	    <Atom>
	      <opr><Rel>sell</Rel></opr>
	      <arg index="1"><Var>merchant</Var></arg>
	      <arg index="2"><Var>person</Var></arg>
	      <arg index="3"> <Var>object</Var></arg>
	    </Atom>
	  </body>
	</Imp>
 </clause>
 
<!-- The third rule is a fact that asserts that
John sells XMLBible to Mary. -->

<clause>
	<Fact>
	  <head>
	    <Atom>
	      <opr><Rel>sell</Rel></opr>
	      <arg index="1"><Ind>John</Ind></arg>
	      <arg index="2"><Ind>Mary</Ind></arg>
	      <arg index="3"><Ind>XMLBible</Ind></arg>
	    </Atom>
	  </head>
	</Fact>
 </clause>
 
<!-- The fourth rule is a fact that asserts that
Mary keeps XMLBible.
 
Observe that this fact is binary - i.e., there are two arguments
for the relation. RDF viewed as a logical knowledge representation
is, likewise, binary, although its arguments have type restrictions,
e.g., the first must be a resource (basically, a URI). Some of the
DTD's on the RuleML website handle URL's/URI's (UR's); see especially
urc-datalog.dtd for inferencing with RDF-like facts -->
 
<clause>
	<Fact>
	  <head>
	    <Atom>
	      <opr><Rel>keep</Rel></opr>
	      <arg index="1"><Ind>Mary</Ind></arg>
	      <arg index="2"><Ind>XMLBible</Ind></arg>
	    </Atom>
	  </head>
	</Fact>
</clause>

</Rulebase>
