Reactive Property over immutable list

package fr.cea.ig.drools;

import org.kie.api.definition.type.PropertyReactive;

import javax.validation.constraints.NotNull;
import java.util.ArrayList;
import java.util.List;

@PropertyReactive
public class Concept {

    private final String name;
    private final List<Concept> concepts;
    private int value;

    @NotNull
    public String getName() {
        return name;
    }

    @NotNull
    public List<Concept> getConcepts() {
        return concepts;
    }


    public void setConcepts(@NotNull List<Concept> c) {
        throw new UnsupportedOperationException();
    }

    public int getValue() {
        return value;
    }

    public void setValue(int value) {
        this.value = value;
    }

    public Concept(final String name, final List<Concept> concepts) {
        this.name = name;
        this.concepts = (concepts == null)? new ArrayList<Concept>() : concepts ;
        value = 0;
    }

    @Override
    public String toString(){
        return String.format("NAME(%s) VALUE(%d)", name, value);
    }
}

package fr.cea.ig.drools;
import fr.cea.ig.drools.Concept;
import java.util.List;

dialect "mvel"


rule "All child concept has a value greater to 0"
    when
    $c : Concept( value == 0 ) @watch( concepts )
    $cl : List(size > 0) from collect( Concept( $c memberOf concepts) )
    forall( Concept( value > 0 ) from $cl )
    then
        modify($c){
            value = 1
        }
end

rule "Concept c1"
    when
    $c : Concept( name == "c1", value == 0 )
    then
        modify($c){
            value = 2
        }
end

rule "Concept c2"
    when
    $c : Concept( name == "c2", value == 0 )
    then
        modify($c){
            value = 3
        }
end