Using Tuples within ModelingToolkit.jl?

Trying to utlized MTK on a larger simulation and having trouble with functions that return tuples that are a function of state. For example:

function foo(x)
    return 1.0,2.0
@register_symbolic foo(x)

function plant(; name)
    @variables x(t)=1 u(t)=0 y(t)=0
    D = Differential(t)

    k1,k2 = foo(x); 

    eqs = [D(x) ~ -x*k1 + u;
              y ~ x*k2]
    ODESystem(eqs, t; name = name)

@named p = plant()

and receiving the error:

ERROR: BoundsError: attempt to access Num at index [2]

I realize this is a trivial example but demonstrates the point. In my actual implementation, the functions that rely on states are complex and have external dependencies (external c-library). I’ve tried writing simple wrappers but at a loss. How do we handle such cases?

I’m not sure it can handle tuple outputs in registered functions. That’s worth an issue. It would need to treat it internally as an array variable.