code: def _opbend_angle_low(a, b, c, deriv=0):
      
      result = _opbend_cos_low(a, b, c, deriv)
      sign = np.sign(np.linalg.det([a, b, c]))
      return _cos_to_angle(result, deriv, sign)
docs: Similar to opbend_angle, but with relative vectors