Error when having differentials w.r.t. species

[TorkelE]

These different cases should now all throw errors.

@reaction_network begin
    @equations D(X) ~ 1.0
    d, X --> 0
end

@reaction_network begin        
    @equations V + X^2 ~ 1.0 - D(V + log(1 + X))
    d, X --> 0
end

@reaction_network begin
    @species Y(t)
    @equations D(Y) ~ 1.0
    d, X --> 0
end

@reaction_network begin
    @equations D(V) ~ 1.0 + D(X)
    d, X --> 0
end