Question on boundary conditions in DiffEqOperators.jl for 2D-case

I just started to play around with DiffEqOperators. I’d like to extend the example at to a 2D case.

Here is the adapted code:

using DiffEqOperators, Plots

f = 5.0
a = -1.0
b = 2.0

nknots = 10
h = 1.0/(nknots+1)
ord_deriv = 2
ord_approx = 2

Δ = CenteredDifference(ord_deriv, ord_approx, h, nknots)
bc = DirichletBC(a, b)

u = (Δ*bc) \ fill(f, (nknots, nknots))

knots = range(h, step=h, length=nknots)
plot(knots, u[nknots÷2, 1:end])
plot!(knots, u[1:end, nknots÷2])

It works somehow - but so to say in 1D Bildschirmfoto 2020-01-07 um 21.39.12

So, my question: How do I define the BC for 2D (or more)? Or maybe there are more things to be changed in the script?