Function cylindrical_mode_matches 

pub(crate) fn cylindrical_mode_matches(mode: Mode, kz: Complex, pol: u8) -> bool

Whether a plane wave of axial wavenumber kz and polarization pol couples to the cylindrical mode: equal pol, a real kz, and kz within 16 ulps of the mode’s.

The tolerance lets a plane basis whose kz values come from arithmetic, such as a normalization, find its cylindrical modes. It is relative, so a change of length units cannot merge two labels.