added another simple test in system_test.py

test/system_test.py

@@ -100,3 +100,20 @@ def test_large_operation_tree():
     output = simulate.run_for(len(random_inputs))
 
     assert simulate.results[num_inputs]['out1'] > sum(random_inputs)
+
+def test_looping_sfg():
+    """A valid looping SFG.
+                +------<constmul2------+
+                |                      |
+                |                      |
+    in1>------add1>----constmul1>------+------out1>
+    """
+    in1 = Input()
+    constmul1 = ConstantMultiplication(1)
+    constmul2 = ConstantMultiplication(1)
+    add1 = in1 + constmul2
+    constmul2.input(0).connect(constmul1)
+    out1 = Output(constmul1)
+
+    return SFG(inputs=[in1], outputs=[out1])
+    
\ No newline at end of file