Add test

Author: YingboMa

test/odesystem.jl

@@ -849,3 +849,17 @@ let
                 D(sys.x) ~ sys.v]
     @test isequal(full_equations(sys_simp), true_eqs)
 end
+
+let
+    @variables t
+    @variables x(t) = 1
+    @variables y(t) = 1
+    @parameters pp = -1
+    der = Differential(t)
+    @named sys4 = ODESystem([der(x) ~ -y; der(y) ~ 1 - y + x], t)
+    as = alias_elimination(sys4)
+    @test length(equations(as)) == 1
+    @test isequal(equations(as)[1].lhs, -der(der(x)))
+    # TODO: maybe do not emit x_t
+    @test_nowarn sys4s = structural_simplify(sys4)
+end