How to Restrict a Child incident not to be a parent incident to other

[harinya]

Hi 
Below is My Requirement,
Can some one please hep
When user tries to add parent for an incident, it needs to check parent incident entered by user is child incident(it should be a child to any incident) or not
If it's child Incident Then it should update the parent incident and provide a message "child incident can't be parent incident" and it abort the action
If it's not child incident to any incident then parent incident column should get updated with given incident and also update the assignment group same as parent incident that user entered

Thanks 

[harinya]

Thanks for Response

The above both solutions i tried but it's not working
please provide other solution
i have tried this script

(function executeRule(current, previous /*null when async*/ ) {

        // Check if the current incident has a parent incident

        var test = current.parent_incident;

        gs.log('incident' + test);

        if (test != '') {

            gs.addErrorMessage('this incident is child incident to other parent' + test.getDisplayValue());

            current.setAbortAction(true);

            gs.addInfoMessage('tested in if');

            return false;

        }

            else {

                gs.addInfoMessage('tested');

               current.assignment_group=parent_incident.assignment_group

                current.update();

            }

        })(current, previous);
it's not working

[James-B]

Both work just fine, however there is an issue with the reference qualifier I gave. The colon is being replaced with HTML code, I have replaced it with an image so it shows the correct code. 

Here is a working xml of the UI policy suggested by "Cris P" which I tested in my PDI.