$session->insertElement_P($object, $relation, [ $type, $value ])
The InsertElement_P() function inserts something into a relationshipset.
In order to access the low-level primitive version of this call, you must invoke it by using the primitive method because the InCharge::session module also has its own variant.
$session->insertElement_P(,"Router", "ComposedOf", [ "OBJREF", "Fan::fan1" ]);