<?xml version="1.0" encoding="UTF-8"?>

<Assert
xmlns="http://www.ruleml.org/0.88/xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.ruleml.org/0.88/xsd http://www.ruleml.org/0.88/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 innerclose="universal">

      <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>