code: def report_collection_diff(from_collection, to_collection, from_id, to_id):
      
      if from_collection == to_collection:
          return None
  
      diff = unified_diff(from_collection, to_collection, fromfile=from_id, tofile=to_id)
      error_message = (
          u"Different tests were collected between {from_id} and {to_id}. "
          u"The difference is:\n"
          u"{diff}"
      ).format(from_id=from_id, to_id=to_id, diff="\n".join(diff))
      msg = "\n".join([x.rstrip() for x in error_message.split("\n")])
      return msg
docs: Report the collected test difference between two nodes.
  
      :returns: detailed message describing the difference between the given
      collections, or None if they are equal.