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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def subpoint(self):
 """Return the latitude and longitude directly beneath this position. Returns a :class:`~skyfield.toposlib.Topos` whose ``longitude`` and ``latitude`` are those of the point on the Earth's surface directly beneath this position, and whose ``elevation`` is the height of this position above the Earth's surface. """

        if self.center != 399:  # TODO: should an __init__() check this?
            raise ValueError("you can only ask for the geographic subpoint"
                             " of a position measured from Earth's center")
        t = self.t
        xyz_au = einsum('ij...,j...->i...', t.M, self.position.au)
        lat, lon, elevation_m = reverse_terra(xyz_au, t.gast)

        # TODO. Move VectorFunction and Topos into this file, since the
        # three kinds of class work together: Topos is-a VF; VF.at() can
        # return a Geocentric position; and Geocentric.subpoint() should
        # return a Topos. I'm deferring the refactoring for now, to get
        # this new feature to users more quickly.
        from .toposlib import Topos
        return Topos(latitude=Angle(radians=lat),
                     longitude=Angle(radians=lon),
                     elevation_m=elevation_m)