<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def root():
 """Placeholder root url for the PCI. Ideally this should never be called! """

    response = {
        "links": {
            "message": "Welcome to the SIP Processing Controller Interface",
            "items": [
                {"href": "{}health".format(request.url)},
                {"href": "{}subarrays".format(request.url)},
                {"href": "{}scheduling_blocks".format(request.url)},
                {"href": "{}processing_blocks".format(request.url)}
            ]
        }
    }
    return response, HTTPStatus.OK