code: def coastal_coords():
      
      coast = list()
      for tile_id in coastal_tile_ids():
          tile_coord = tile_id_to_coord(tile_id)
          for edge_coord in coastal_edges(tile_id):
              dirn = tile_edge_offset_to_direction(edge_coord - tile_coord)
              if tile_id_in_direction(tile_id, dirn) is None:
                  coast.append((tile_id, dirn))
      # logging.debug('coast={}'.format(coast))
      return coast
docs: A coastal coord is a 2-tuple: (tile id, direction).
  
      An edge is coastal if it is on the grid's border.
  
      :return: list( (tile_id, direction) )