code: def extract_segment_types(urml_document_element, namespace):
      
      segment_types = \
          {namespace+':'+seg.attrib['id']: seg.tag
           for seg in urml_document_element.iter('nucleus', 'satellite')}
  
      for seg in urml_document_element.iter('segment'):
          seg_id = namespace+':'+seg.attrib['id']
          if seg_id not in segment_types:
              segment_types[seg_id] = 'isolated'
      return segment_types
docs: Return a map from segment node IDs to their segment type
      ('nucleus', 'satellite' or 'isolated').