class AMultilayer: public TObject

 Copied from tmm.is_forward_angle
 if a wave is traveling at angle theta from normal in a medium with index n,
 calculate whether or not this is the forward-traveling wave (i.e., the one
 going from front to back of the stack, like the incoming or outgoing waves,
 but unlike the reflected wave). For real n & theta, the criterion is simply
 -pi/2 < theta < pi/2, but for complex n & theta, it's more complicated.
 See https://arxiv.org/abs/1603.02720 appendix D. If theta is the forward
 angle, then (pi-theta) is the backward angle and vice-versa.

 Copied from tmm.list_snell
 return list of angle theta in each layer based on angle th_0 in layer 0,
 using Snell's law. n_list is index of refraction of each layer. Note that
 "angles" may be complex!!

 ----------------- Top layer
 ----------------- 1st layer
 ----------------- 2nd layer

 ----------------- <------------- Insert a new layer here
 ----------------- Bottom layer

 Copied from tmm.ch_tmm
 Main "coherent transfer matrix method" calc. Given parameters of a stack,
 calculates everything you could ever want to know about how light
 propagates in it. (If performance is an issue, you can delete some of the
 calculations without affecting the rest.)

 pol is light polarization, "s" or "p".

 n_list is the list of refractive indices, in the order that the light would
 pass through them. The 0'th element of the list should be the semi-infinite
 medium from which the light enters, the last element should be the semi-
 infinite medium to which the light exits (if any exits).

 th_0 is the angle of incidence: 0 for normal, pi/2 for glancing.
 Remember, for a dissipative incoming medium (n_list[0] is not real), th_0
 should be complex so that n0 sin(th0) is real (intensity is constant as
 a function of lateral position).

 d_list is the list of layer thicknesses (front to back). Should correspond
 one-to-one with elements of n_list. First and last elements should be
 "inf".

 lam_vac is vacuum wavelength of the light.

 Note that having n larger than 1 frequently decreases the total
 performance. Use this method only when you feed a very long vector.