def get_edge( tile, edge ):
    """
    This function returns the specific edge of the given tile.
    """
    if edge == 'top':
        return tile[0,:]
    elif edge == 'bottom':
        return tile[-1,:]
    elif edge == 'left':
        return tile[:,0]
    elif edge == 'right':
        return tile[:,-1]
    else:
        raise ValueError("The arg 'edge' should be top, bottom, left or right!")