Boundary conditions that depend on the horizontal wave number

[BenMql]

coral handles linear, homogeneous, decoupled boundary conditions via Galerkin basis recombination.
In the current state, these boundary conditions are identical for all horizontal wave numbers, such as Dirichlet ($\phi=0$) and Von Neuman ($\partial_{z}\phi=0$) boundary conditions.
In some instances, see e.g. equation (B7) in Miquel, Ellison, Calkins, Julien, Knobloch 2026, the formulation of the problem is conducive to boundary conditions that depend on the horizontal wavenumber, e.g.

$$\left( \partial_3 + a \partial_2\right) \psi = 0$$

which is readily expressed by introducing the horizontal wave number $k_2$:

$$\left( \partial_3 + iak_2 \right) \psi = 0$$

This demands that Galerkin stencils vary with the horizontal wavenumber $k_2$.