Function tr 

pub fn tr(
    matrices: [MatRef<'_, Complex>; 2],
    incident: DMatrix<Complex>,
    ports: TrPorts,
    fixed_q: bool,
) -> Result<TrResidual>

Transmittance and reflectance of S-matrix columns between two plane-wave ports.

TrResidual::value has the rows (transmittance, reflectance) and one column per illumination; the residual keeps it because its pullback reads it. The pullback includes the interference of incident and reflected waves in a lossy medium. With fixed_q, it holds the transverse wavevectors fixed and supports exactly normal incidence; otherwise derivatives preserve the distinct diffraction-group topology.

Upstream: treams.SMatrices.tr.