I am modelling a PDE describing the deformations of a beam, this has the form of a second order PDE in time along with an integral condition on the beam’s strain. I am attempting to discretise this using ModelingToolkit and the MethodOfLines packages. The following is the code I am using:

```
using Plots, MethodOfLines, DomainSets, ModelingToolkit
@variables u(..) f(..)
@parameters t θ λ P
dθ = Differential(θ)
dθθ = Differential(θ)^2
dθθθθ = Differential(θ)^4
dt = Differential(t)
dtt = Differential(t)^2
function δ(θ)
return 1/1000*sqrt(2π) * exp(-θ^2/(2*1000^2))
end
Iθ = Integral(θ in DomainSets.ClosedInterval(-1.0,1.0))
eq = [
dtt(u(t,θ)) ~ -dθθθθ(u(t,θ)) + λ^2 * dθ(f(t,θ) - u(t,θ) +0.5*dθ(u(t,θ))^2*dθ(u(t,θ)))+λ^2 * (f(t,θ) - u(t,θ) +0.5*dθ(u(t,θ))^2)+P*δ(θ), 0 ~ Iθ(f(t,θ))
]
bcs = [
u(t,-1.0) ~ 0.0, u(t,1.0) ~ 0.0,
dθ(dθ(u(t,-1.0))) ~ 0.0, dθ(dθ(u(t,1.0))) ~ 0.0,
dθ(f(t,-1.0)) - dθ(u(t,-1.0)) ~ 0.0, dθ(f(t,1.0)) - dθ(u(t,1.0)) ~ 0.0,
u(0.0,θ) ~ 0.0, dt(u(0.0,θ)) ~ 0.0
]
domains = [t ∈ (0.0,50.0),
θ ∈ (-1.0,1.0)]
@named pde_sys = PDESystem(eq, bcs, domains, [t,θ], [u(t,θ),f(t,θ)])
discretisation = MOLFiniteDifference([θ => 100],t)
prob = MethodOfLines.discretize(pde_sys, discretisation)
```

But this is giving the error:

ERROR: UndefVarError: `update_varmap!`

not defined

I have looked through the docs for MethodOfLines and cannot find where this function should be defined?