`abstract`

A marker expression to denote that the attribute's derivation must be specified on concrete sub-classes (or one of their superclasses).
If one or more attributes in a rule class is marked abstract, then the CER rule set validator will insist that the class itself is marked abstract="true", and prevent the rule class from being used in any create expressions.
<?xml version="1.0" encoding="UTF-8"?>
<RuleSet name="Example_abstract"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation=
"http://www.curamsoftware.com/CreoleRulesSchema.xsd">

  <!-- Base class for all types of benefit.
       Every concrete subclass has its own
       calculation of "name" and "isEligible". -->
  <Class name="Benefit" abstract="true">
    <Initialization>
      <!-- The person for which benefit eligibility
        is being determined. -->
      <Attribute name="person">
        <type>
          <ruleclass name="Person"/>
        </type>
      </Attribute>
    </Initialization>


    <!-- The name of this type of benefit -->
    <Attribute name="name">
      <type>
        <javaclass name="String"/>
      </type>
      <derivation>
        <abstract/>
      </derivation>
    </Attribute>

    <!-- Whether the person is eligible for this benefit. -->
    <Attribute name="isEligible">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <abstract/>
      </derivation>
    </Attribute>

  </Class>

  <!-- A concrete subclass of Benefit.
       Contains concrete derivations for the inherited
       abstract attributes. -->
  <Class name="MedicalBenefit" extends="Benefit">
    <Attribute name="name">
      <type>
        <javaclass name="String"/>
      </type>
      <derivation>
        <String value="Medical Benefit"/>
      </derivation>
    </Attribute>
    <Attribute name="isEligible">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <all>
          <fixedlist>
            <listof>
              <javaclass name="Boolean"/>
            </listof>
            <members>
              <!-- NB the person attribute is inherited from Benefit
 -->
              <reference attribute="isPoor">
                <reference attribute="person"/>
              </reference>
              <reference attribute="isSick">
                <reference attribute="person"/>
              </reference>
            </members>
          </fixedlist>

        </all>

      </derivation>
    </Attribute>
  </Class>


  <!-- Another concrete subclass of Benefit,
    with different concrete derivations for the inherited
    abstract attributes. -->
  <Class name="NeedyBenefit" extends="Benefit">
    <Attribute name="name">
      <type>
        <javaclass name="String"/>
      </type>
      <derivation>
        <String value="Medical Benefit"/>
      </derivation>
    </Attribute>
    <Attribute name="isEligible">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <all>
          <fixedlist>
            <listof>
              <javaclass name="Boolean"/>
            </listof>
            <members>
              <reference attribute="isPoor">
                <reference attribute="person"/>
              </reference>
              <any>
                <fixedlist>
                  <listof>
                    <javaclass name="Boolean"/>
                  </listof>
                  <members>
                    <reference attribute="isHungry">
                      <reference attribute="person"/>
                    </reference>
                    <reference attribute="isDeprived">
                      <reference attribute="person"/>
                    </reference>
                  </members>
                </fixedlist>
              </any>
            </members>
          </fixedlist>

        </all>

      </derivation>
    </Attribute>
  </Class>

  <Class name="Person">

    <Attribute name="isPoor">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <specified/>
      </derivation>
    </Attribute>

    <Attribute name="isSick">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <specified/>
      </derivation>
    </Attribute>

    <Attribute name="isHungry">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <specified/>
      </derivation>
    </Attribute>

    <Attribute name="isDeprived">
      <type>
        <javaclass name="Boolean"/>
      </type>
      <derivation>
        <specified/>
      </derivation>
    </Attribute>


    <!-- A list of all the benefits for
         which the person is being assessed.  -->
    <Attribute name="allBenefits">
      <type>
        <javaclass name="List">
          <ruleclass name="Benefit"/>
        </javaclass>
      </type>
      <derivation>
        <fixedlist>
          <listof>
            <ruleclass name="Benefit"/>
          </listof>
          <members>
            <!-- Create instances of the concrete rule classes -->
            <create ruleclass="MedicalBenefit">
              <this/>
            </create>
            <create ruleclass="NeedyBenefit">
              <this/>
            </create>
          </members>
        </fixedlist>

      </derivation>
    </Attribute>

    <!-- The benefits for which this person
         is eligible.

         Note that the list is of the abstract
         rule class "Benefit", but that each
         concrete instance determines its
         eligibility in its own way.    -->
    <Attribute name="eligibleBenefits">
      <type>
        <javaclass name="List">
          <ruleclass name="Benefit"/>
        </javaclass>
      </type>
      <derivation>
        <filter>
          <list>
            <reference attribute="allBenefits"/>
          </list>
          <listitemexpression>
            <reference attribute="isEligible">
              <current/>
            </reference>
          </listitemexpression>
        </filter>
      </derivation>
    </Attribute>

  </Class>

</RuleSet>