Clean up and export flatten

Author: YingboMa

src/systems/reduction.jl

@@ -1,4 +1,4 @@
-export alias_elimination
+export alias_elimination, flatten
 
 function flatten(sys::ODESystem)
     if isempty(sys.systems)

test/reduction.jl

@@ -109,7 +109,7 @@ let
         x ~ y
     ];
     sys = ODESystem(eqs, t, [x], []);
-    asys = alias_elimination(ModelingToolkit.flatten(sys))
+    asys = alias_elimination(flatten(sys))
 
     test_equal.(asys.eqs, [D(x) ~ 2x])
     test_equal.(asys.observed, [y ~ x])
@@ -119,27 +119,18 @@ end
 let
     @parameters t
     @derivatives D'~t
-
-    ## Define a system
-
     @variables x(t), u(t), y(t)
     @parameters a, b, c, d
     ol = ODESystem([D(x) ~ a * x + b * u, y ~ c * x], t, name=:ol)
-
-    ## Define another system that is purely observing
-
     @variables u_c(t), y_c(t)
     @parameters k_P
     pc = ODESystem(Equation[], t, pins=[y_c], observed = [u_c ~ k_P * y_c], name=:pc)
-
-    ## Connect them
-
     connections = [
                    ol.u ~ pc.u_c,
                    y_c ~ ol.y
                   ]
+    connected = ODESystem(connections, t, systems=[ol, pc])
 
-    connected = MTK.ODESystem(connections, t, systems=[ol, pc])
-
-    @test_nowarn MTK.flatten(connected)
+    @test equations(connected) isa Vector{Equation}
+    @test_nowarn flatten(connected)
 end