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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 _at(self, t):
 """Compute the GCRS position and velocity of this Topos at time `t`."""

        pos, vel = terra(self.latitude.radians, self.longitude.radians,
                         self.elevation.au, t.gast)
        pos = einsum('ij...,j...->i...', t.MT, pos)
        vel = einsum('ij...,j...->i...', t.MT, vel)
        if self.x:
            R = rot_y(self.x * ASEC2RAD)
            pos = einsum('ij...,j...->i...', R, pos)
        if self.y:
            R = rot_x(self.y * ASEC2RAD)
            pos = einsum('ij...,j...->i...', R, pos)
        # TODO: also rotate velocity

        return pos, vel, pos, None