Hi, I essentially want to solve an **easier** version of the copy-paste example provided in the docs here. Instead of connecting the two lorenz systems via an algebraic equality, I want to add the state of one system as a time-varying input to another system. I tried to code this below:

```
@parameters t σ ρ β
@parameters inp(t) #extra input to the first lorenz system I've added
@variables x(t) y(t) z(t)
@derivatives D'~t
eqs1 = [D(x) ~ σ*(y-x) + inp,
D(y) ~ x*(ρ-z)-y,
D(z) ~ x*y - β*z]
eqs2 = [D(x) ~ σ*(y-x),
D(y) ~ x*(ρ-z)-y,
D(z) ~ x*y - β*z]
lorenz1 = ODESystem(eqs1,name=:lorenz1)
lorenz2 = ODESystem(eqs2,name=:lorenz2)
connections = [lorenz1.inp ~ lorenz2.x]
connected = ODESystem(connections,t,[],[],systems=[lorenz1,lorenz2])
u0 = [lorenz1.x => 1.0,
lorenz1.y => 0.0,
lorenz1.z => 0.0,
lorenz2.x => 0.0,
lorenz2.y => 1.0,
lorenz2.z => 0.0]
p = [lorenz1.σ => 10.0,
lorenz1.ρ => 28.0,
lorenz1.β => 8/3,
lorenz2.σ => 10.0,
lorenz2.ρ => 28.0,
lorenz2.β => 8/3,
lorenz1.inp => 0.]
tspan = (0.0,100.0)
prob = ODEProblem(connected,u0,tspan,p)
sol = solve(prob,Rodas5())
```

However, I can’t get it to work. For the above code, I get the error.

ERROR: Only semi-explicit constant mass matrices are currently supported. Faulty equation: Equation(lorenz1₊inp, lorenz2₊x(t)).

Any idea how to get this to work? Thank you very much in advance!