code: def ApproximateDistanceBetweenStops(stop1, stop2):
    
    if (stop1.stop_lat is None or stop1.stop_lon is None or
        stop2.stop_lat is None or stop2.stop_lon is None):
      return None
    return ApproximateDistance(stop1.stop_lat, stop1.stop_lon,
                               stop2.stop_lat, stop2.stop_lon)
docs: Compute approximate distance between two stops in meters. Assumes the
    Earth is a sphere.