def derivative(self, T, n=<NUM_LIT:1>):
seg_idx, t = self.T2t(T)<EOL>seg = self._segments[seg_idx]<EOL>return seg.derivative(t, n=n)/seg.length()**n<EOL>
returns the tangent vector of the Path at T (centered at the origin
        and expressed as a complex number).
        Note: Bezier curves can have points where their derivative vanishes.
        If you are interested in the tangent direction, use unit_tangent()
        method instead.