<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def calc_time(lower_bound, upper_bound, latitude, longitude, attribute, value, altitude=0, pressure=101325, temperature=12, horizon='+0:00', xtol=1.0e-12):
 """ Calculate the time between lower_bound and upper_bound where the attribute is equal to value. Uses PyEphem for solar position calculations. Parameters lower_bound : datetime.datetime upper_bound : datetime.datetime latitude : float longitude : float attribute : str The attribute of a pyephem.Sun object that you want to solve for. Likely options are 'alt' and 'az' (which must be given in radians). value : int or float The value of the attribute to solve for altitude : float, default 0 Distance above sea level. pressure : int or float, optional, default 101325 Air pressure in Pascals. Set to 0 for no atmospheric correction. temperature : int or float, optional, default 12 Air temperature in degrees C. horizon : string, optional, default '+0:00' arc degrees:arc minutes from geometrical horizon for sunrise and sunset, e.g., horizon='+0:00' to use sun center crossing the geometrical horizon to define sunrise and sunset, horizon='-0:34' for when the sun's upper edge crosses the geometrical horizon xtol : float, optional, default 1.0e-12 The allowed error in the result from value Returns ------- datetime.datetime Raises ------ ValueError If the value is not contained between the bounds. AttributeError If the given attribute is not an attribute of a PyEphem.Sun object. """


    try:
        import scipy.optimize as so
    except ImportError:
        raise ImportError('The calc_time function requires scipy')

    obs, sun = _ephem_setup(latitude, longitude, altitude,
                            pressure, temperature, horizon)

    def compute_attr(thetime, target, attr):
        obs.date = thetime
        sun.compute(obs)
        return getattr(sun, attr) - target

    lb = datetime_to_djd(lower_bound)
    ub = datetime_to_djd(upper_bound)

    djd_root = so.brentq(compute_attr, lb, ub,
                         (value, attribute), xtol=xtol)

    return djd_to_datetime(djd_root)