I’m working with ModellingToolkit and DiffEqOperators to solve a set of PDEs, and I wanted to test out a simpler problem before I jumped into a more complicated problem. I followed the heat equation example and that worked, so I next wanted to try 1D linear advection with periodic boundary conditions, but I’m having some issues with the boundary conditions:

```
@parameters t x
@variables u(..)
@derivatives Dt'~t
@derivatives Dx'~x
a = 1
advection_eq = Dt(u(t, x)) ~ -a * Dx(u(t, x))
advection_bcs = [
u(0, x) ~ sinpi(x),
PeriodicBC(Float64)
]
domains = [
t ∈ IntervalDomain(0.0, 1.0),
x ∈ IntervalDomain(0.0, 1.0)
]
advection_pdesys = PDESystem(advection_eq, advection_bcs, domains, [t, x], [u])
advection_dx = 0.01
advection_order = 2
advection_discretization = MOLFiniteDifference(advection_dx, advection_order)
advection_prob = ModelingToolkit.discretize(advection_pdesys, advection_discretization)
advection_sol = solve(advection_prob, Tsit5(), saveat = 0.1)
```

This errors because `type PeriodicBC has no field lhs`

, so I’m definitely using this incorrectly, so I was wondering what the intended way to use the periodic boundary condition was (and for that matter, other non-dirichlet boundary conditions)

I’ve also tried `u(t, 0) ~ u(t, 1)`

but that gives me either an undefined reference error or `UndefVarError: u not defined`