def opbend_angle(rs, deriv=<NUM_LIT:0>):
return _opbend_transform(rs, _opbend_angle_low, deriv)<EOL>
Compute the angle between the vector rs[0], rs[3] and the plane rs[0], rs[1], rs[2]

       The sign convention is as follows: positive if rs[3] lies in the space
       above plane rs[0], rs[1], rs[2] and negative if rs[3] lies below. Above
       is defined by right hand rule from rs[0]-rs[1] to rs[0]-rs[2].

       Arguments:
        | ``rs``  --  four numpy array with three elements
        | ``deriv``  --  the derivatives to be computed: 0, 1 or 2 [default=0]

       When no derivatives are computed a tuple with a single result is returned.