Fix order lowering

Fix #708

Author: YingboMa

src/systems/diffeqs/first_order_transform.jl

@@ -15,11 +15,9 @@ function ode_order_lowering(eqs, iv, states)
     diff_eqs = Equation[]
     diff_vars = []
     alge_eqs = Equation[]
-    alge_vars = []
 
     for (i, (eq, ss)) ∈ enumerate(zip(eqs, states))
         if _iszero(eq.lhs)
-            push!(alge_vars, ss)
             push!(alge_eqs, eq)
         else
             var, maxorder = var_from_nested_derivative(eq.lhs)
@@ -45,5 +43,5 @@ function ode_order_lowering(eqs, iv, states)
     end
 
     # we want to order the equations and variables to be `(diff, alge)`
-    return (vcat(diff_eqs, alge_eqs), vcat(diff_vars, alge_vars))
+    return (vcat(diff_eqs, alge_eqs), vcat(diff_vars, setdiff(states, diff_vars)))
 end