<?xml version="1.0" encoding="UTF-8"?>
<Assert xmlns="http://www.ruleml.org/0.89/xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.ruleml.org/0.89/xsd http://www.ruleml.org/0.89/xsd/datalog.xsd">
	<!-- start XML comment ...

Note: This is a compact (i.e. minimally-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:

Implies~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          *                         *
     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 -->
	<content>
		<And mapClosure="universal" mapDirection="bidirectional">
			<formula>
				<Implies>
					<!-- explicit 'And' -->
					<body>
						<And>
							<formula>
								<Atom>
									<opr>
										<Rel>buy</Rel>
									</opr>
									<arg index="1">
										<Var>person</Var>
									</arg>
									<arg index="2">
										<Var>merchant</Var>
									</arg>
									<arg index="3">
										<Var>object</Var>
									</arg>
								</Atom>
							</formula>
							<formula>
								<Atom>
									<opr>
										<Rel>keep</Rel>
									</opr>
									<arg index="1">
										<Var>person</Var>
									</arg>
									<arg index="2">
										<Var>object</Var>
									</arg>
								</Atom>
							</formula>
						</And>
					</body>
					<head>
						<Atom>
							<opr>
								<Rel>own</Rel>
							</opr>
							<arg index="1">
								<Var>person</Var>
							</arg>
							<arg index="2">
								<Var>object</Var>
							</arg>
						</Atom>
					</head>
				</Implies>
			</formula>
			<!-- The second rule implies that a person buys an object from a merchant
if the merchant sells the object to the person. -->
			<formula>
				<Implies>
					<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>
					<head>
						<Atom>
							<opr>
								<Rel>buy</Rel>
							</opr>
							<arg index="1">
								<Var>person</Var>
							</arg>
							<arg index="2">
								<Var>merchant</Var>
							</arg>
							<arg index="3">
								<Var>object</Var>
							</arg>
						</Atom>
					</head>
				</Implies>
			</formula>
			<!-- The third rule is a fact that asserts that
John sells XMLBible to Mary. -->
			<formula>
				<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>
			</formula>
			<!-- 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 -->
			<formula>
				<Atom>
					<opr>
						<Rel>keep</Rel>
					</opr>
					<arg index="1">
						<Ind>Mary</Ind>
					</arg>
					<arg index="2">
						<Ind>XMLBible</Ind>
					</arg>
				</Atom>
			</formula>
		</And>
	</content>
</Assert>

