def unit_tangent(self, T):
        """returns the unit tangent vector of the Path at T (centered at the
        origin and expressed as a complex number).  If the tangent vector's
        magnitude is zero, this method will find the limit of
        self.derivative(tau)/abs(self.derivative(tau)) as tau approaches T."""
        seg_idx, t = self.T2t(T)
        return self._segments[seg_idx].unit_tangent(t)
returns the unit tangent vector of the Path at T (centered at the
        origin and expressed as a complex number).  If the tangent vector's
        magnitude is zero, this method will find the limit of
        self.derivative(tau)/abs(self.derivative(tau)) as tau approaches T.