title: Assigning multiple values to a property
body: <p>I am using rowlex in my project. I have a property assigned to an individual in my RDF file, which has a value. For example for individual 'Student', there is a property 'isMemberOf', with value of a class uri 'class00021'.
  Then I want to add a second value to this property. For instance a 'Project' value with uri 'proj000052'.</p>
  
  <p>The problem appears here: after adding the second value, first value is thrown out of property 'isMemberOf', even out of its individual (student), and is stored as a new individual.</p>
  
  <p>The code I used for this operation is like this:</p>
  
  <pre><code>//Add a class to a student 
  public void Add_Class
      (string uri_stu, string uri_class)
  {        
          //Open RDF
          RdfDocument rdfDoc = new RdfDocument(@"RDF_Repository\RDF_Student.rdf");
          //Find the student
          //Student student = new Student(uri_stu, rdfDoc);
          Student student = (Student)rdfDoc.GetIndividual(uri_stu);
          //Add a class
          student.studyMemberOf = new ClassOfCourse(uri_class, rdfDoc);
          rdfDoc.ExportToRdfXml(@"RDF_Repository\RDF_Student.rdf");
  }
  
  
  //Add a project to a student 
  public void Add_Project
      (string uri_stu, string uri_proj)
  {
          //Open RDF
          RdfDocument rdfDoc = new RdfDocument(@"RDF_Repository\RDF_Student.rdf");
          //Find the student
          Student student = (Student)rdfDoc.GetIndividual(uri_stu);
          //Add a project                        
          student.studyMemberOf = new Project(uri_proj, rdfDoc);
          rdfDoc.ExportToRdfXml(@"RDF_Repository\RDF_Student.rdf");
  }
  </code></pre>
  
  <p>The resulted RDF is like this:</p>
  
  <pre><code>&lt;?xml version="1.0"?&gt;
  &lt;rdf:RDF xmlns:Ontologyowl="http://www.owl-ontologies.com/Ontology1243411901.owl#" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"&gt;
      &lt;Ontologyowl:Student rdf:about="stu000012"&gt;
          &lt;Ontologyowl:studyMemberOf&gt;
              &lt;Ontologyowl:Project rdf:about="proj000052"/&gt;
          &lt;/Ontologyowl:studyMemberOf&gt;
      &lt;/Ontologyowl:Student&gt;
      &lt;Ontologyowl:ClassOfCourse rdf:about="class000021"/&gt;
  &lt;/rdf:RDF&gt;
  </code></pre>
  
  <p>... and if we continue adding, the previous property will be thrown out.
  So how can I overcome this problem?</p>