code: public void setRelationshipType(RelationshipType relationshipType) {
  		replace(RELTYPE, (relationshipType == null) ? null : relationshipType.getValue());
  	}
docs: <p>
  Sets the RELTYPE (relationship type) parameter value.
  </p>
  <p>
  This parameter is used by the {@link RelatedTo} property. It defines the
  kind of relationship the property is describing (for example, a "child"
  relationship).
  </p>
  @param relationshipType the relationship type or null to remove
  @see <a href="http://tools.ietf.org/html/rfc5545#page-25">RFC 5545
  p.25</a>