code: def coastal_edges(tile_id):
      
      edges = list()
      tile_coord = tile_id_to_coord(tile_id)
      for edge_coord in edges_touching_tile(tile_id):
          dirn = tile_edge_offset_to_direction(edge_coord - tile_coord)
          if tile_id_in_direction(tile_id, dirn) is None:
              edges.append(edge_coord)
      return edges
docs: Returns a list of coastal edge coordinate.
  
      An edge is coastal if it is on the grid's border.
      :return: list(int)