Fix typo in alias elimination

Author: Keno

src/systems/alias_elimination.jl

@@ -258,7 +258,7 @@ function alias_eliminate_graph!(graph, var_to_diff, mm_orig::SparseMatrixCLIL)
     end
 
     # kind of like the backward substitution
-    lss!(ei::Integer) = locally_structure_simplify!((@view mm[ei, :]), pivots[ei], ag, diff_to_var[pivots[ei]] == 0)
+    lss!(ei::Integer) = locally_structure_simplify!((@view mm[ei, :]), pivots[ei], ag, isnothing(diff_to_var[pivots[ei]]))
 
     # Step 2.1: Go backwards, collecting eliminated variables and substituting
     #         alias as we go.

test/reduction.jl

@@ -49,8 +49,8 @@ reduced_eqs = [
 test_equal.(equations(lorenz1_aliased), reduced_eqs)
 @test isempty(setdiff(states(lorenz1_aliased), [x, y, z]))
 test_equal.(observed(lorenz1_aliased), [
+                                        u ~ 0
                                         z ~ x - y
-                                        u ~ -0.0
                                         a ~ -z
                                        ])