def bbox(tile_coord, grid):
    """
    >>> from mapproxy.grid import tile_grid
    >>> grid = tile_grid(4326, bbox=(0, -15, 10, -5))
    >>> bbox((0, 0, 0), grid)
    '0.00000000,-15.00000000,10.00000000,-5.00000000'
    >>> bbox((0, 0, 1), grid)
    '0.00000000,-15.00000000,5.00000000,-10.00000000'

    >>> grid = tile_grid(4326, bbox=(0, -15, 10, -5), origin='nw')
    >>> bbox((0, 0, 1), grid)
    '0.00000000,-10.00000000,5.00000000,-5.00000000'
    """
    return '%.8f,%.8f,%.8f,%.8f' % grid.tile_bbox(tile_coord)
