diff --git a/plesna/graph/__init__.py b/plesna/graph/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/plesna/graph.py b/plesna/graph/graph.py
similarity index 100%
rename from plesna/graph.py
rename to plesna/graph/graph.py
diff --git a/plesna/graph_set.py b/plesna/graph/graph_set.py
similarity index 94%
rename from plesna/graph_set.py
rename to plesna/graph/graph_set.py
index 71e3f93..a52fdfc 100644
--- a/plesna/graph_set.py
+++ b/plesna/graph/graph_set.py
@@ -31,3 +31,6 @@ class GraphSet:
     @property
     def node_sets(self):
         return self._node_sets
+
+    def is_valid_dag(self):
+        pass
diff --git a/tests/graphs/test_graph.py b/tests/graphs/test_graph.py
index 8f2c406..24f5eb8 100644
--- a/tests/graphs/test_graph.py
+++ b/tests/graphs/test_graph.py
@@ -1,6 +1,6 @@
 import pytest
 
-from plesna.graph import Edge, Graph, Node
+from plesna.graph.graph import Edge, Graph, Node
 
 
 def test_append_nodess():
diff --git a/tests/graphs/test_graph_set.py b/tests/graphs/test_graph_set.py
index b4c5fda..c402202 100644
--- a/tests/graphs/test_graph_set.py
+++ b/tests/graphs/test_graph_set.py
@@ -1,4 +1,4 @@
-from plesna.graph_set import EdgeOnSet, GraphSet, Node
+from plesna.graph.graph_set import EdgeOnSet, GraphSet, Node
 
 
 def test_init():