I am trying out the `ModelingToolkit.jl`

package and I get this weird error when defing the classical harmonic oscillator with the following code

```
using ModelingToolkit,
@variables t x(t)
@parameters ω₀
Dₜ = Differential(t)
@named sys = ODESystem([Dₜ(Dₜ(x)) + ω₀^2*x ~ 0])
```

It produces the error

```
ArgumentError: Variable x(t) is not a function of independent variable (ω₀^2)*x(t).
```

I do not understand the where the error comes from. Just putting the second term on the other side gets rid of the error, i.e.

```
using ModelingToolkit,
@variables t x(t)
@parameters ω₀
Dₜ = Differential(t)
@named sys = ODESystem([Dₜ(Dₜ(x)) ~ - ω₀^2*x])
```

runs just fine. My apologies if this error comes from my lack of understanding how the package must be used.