state_selection: Correct is_not_present condition

The is_present condition is supposed to be equivalent to the variable
not appearing in the system at all (for example, it could appear as
`a*D(v)`, where `a` ends up aliased to `0`, meaning that we don't
notice that it does not appear in the system until we've performed
alias analysis. In these kinds of situations, a variable is present
in the system if, either:

1. It appears in the graph (i.e. it's used in the equations), or
2. It is irreducible (and was in the alias graph).

The condition was trying to check this, but didn't quite get the
negations correct, resulting in sub-optimal state selection.

Author: Keno

src/structural_transformation/partial_state_selection.jl

@@ -277,22 +277,25 @@ function dummy_derivative_graph!(structure::SystemStructure, var_eq_matching, ja
                 end
                 x = diff_to_var[x]
                 x === nothing && break
+                isempty(𝑑neighbors(graph, x)) && continue
                 if !haskey(ag, x)
                     isred = false
                 end
             end
             isred
         end
         irreducible_set = BitSet()
-        for (k, (_, v)) in ag
+        for (k, (c, v)) in ag
             isreducible(k) || push!(irreducible_set, k)
             isreducible(k) || push!(irreducible_set, k)
+            iszero(c) && continue
             push!(irreducible_set, v)
         end
     end
 
     is_not_present = v -> isempty(𝑑neighbors(graph, v)) &&
-        (ag === nothing || (haskey(ag, v) && !(v in irreducible_set)))
+        (ag === nothing || !haskey(ag, v) || !(v in irreducible_set))
+
     # Derivatives that are either in the dummy derivatives set or ended up not
     # participating in the system at all are not considered differential
     is_some_diff = let dummy_derivatives_set = dummy_derivatives_set