diff --git a/src/main/java/edu/kit/formal/gui/controller/DebuggerMainWindowController.java b/src/main/java/edu/kit/formal/gui/controller/DebuggerMainWindowController.java
index 0aed81137fe2492843752a739cc1baf00e207159..94a3a039bd2770e0a646e89ade7b71dd73272d73 100644
--- a/src/main/java/edu/kit/formal/gui/controller/DebuggerMainWindowController.java
+++ b/src/main/java/edu/kit/formal/gui/controller/DebuggerMainWindowController.java
@@ -4,8 +4,12 @@ import de.uka.ilkd.key.proof.init.ProofInputException;
 import de.uka.ilkd.key.speclang.Contract;
 import edu.kit.formal.gui.controls.*;
 import edu.kit.formal.gui.model.RootModel;
-import edu.kit.formal.interpreter.*;
+import edu.kit.formal.interpreter.Interpreter;
+import edu.kit.formal.interpreter.InterpreterBuilder;
+import edu.kit.formal.interpreter.KeYProofFacade;
 import edu.kit.formal.interpreter.data.KeyData;
+import edu.kit.formal.interpreter.graphs.PTreeNode;
+import edu.kit.formal.interpreter.graphs.ProofTreeController;
 import edu.kit.formal.proofscriptparser.Facade;
 import edu.kit.formal.proofscriptparser.ast.ProofScript;
 import javafx.beans.property.SimpleBooleanProperty;
diff --git a/src/main/java/edu/kit/formal/interpreter/InterpretingService.java b/src/main/java/edu/kit/formal/interpreter/InterpretingService.java
index 2cd4f245a95bc5dceba299f7f1c148525bbc1447..41da703d32c06aadadcff5d7d7a6d2aec6e83702 100644
--- a/src/main/java/edu/kit/formal/interpreter/InterpretingService.java
+++ b/src/main/java/edu/kit/formal/interpreter/InterpretingService.java
@@ -14,7 +14,7 @@ import javafx.concurrent.Task;
  *
  * @author A. Weigl
  */
-class InterpretingService extends Service<State<KeyData>> {
+public class InterpretingService extends Service<State<KeyData>> {
 
     /**
      * The interpreter (with teh appropriate KeY state) that is used to traverse and execute the script
@@ -31,7 +31,7 @@ class InterpretingService extends Service<State<KeyData>> {
      */
     private PuppetMaster blocker;
 
-    InterpretingService(PuppetMaster blocker) {
+    public InterpretingService(PuppetMaster blocker) {
         this.blocker = blocker;
     }
 
diff --git a/src/main/java/edu/kit/formal/interpreter/NodeAddedEvent.java b/src/main/java/edu/kit/formal/interpreter/NodeAddedEvent.java
index b1e047464dcdff3c5a6f1af52e09af57ecc7bab7..a3bdc4644f0af5e7b3804759437bbcfb414d5ce5 100644
--- a/src/main/java/edu/kit/formal/interpreter/NodeAddedEvent.java
+++ b/src/main/java/edu/kit/formal/interpreter/NodeAddedEvent.java
@@ -1,5 +1,6 @@
 package edu.kit.formal.interpreter;
 
+import edu.kit.formal.interpreter.graphs.PTreeNode;
 import lombok.Getter;
 
 /**
diff --git a/src/main/java/edu/kit/formal/interpreter/exceptions/StateGraphException.java b/src/main/java/edu/kit/formal/interpreter/exceptions/StateGraphException.java
new file mode 100644
index 0000000000000000000000000000000000000000..47daed562aeb92d13954e9c0ba352f08f3b27018
--- /dev/null
+++ b/src/main/java/edu/kit/formal/interpreter/exceptions/StateGraphException.java
@@ -0,0 +1,26 @@
+package edu.kit.formal.interpreter.exceptions;
+
+/**
+ * Exception that is thrown if State graph could not be build properly
+ */
+public class StateGraphException extends RuntimeException {
+    public StateGraphException() {
+        super();
+    }
+
+    public StateGraphException(String message) {
+        super(message);
+    }
+
+    public StateGraphException(String message, Throwable cause) {
+        super(message, cause);
+    }
+
+    public StateGraphException(Throwable cause) {
+        super(cause);
+    }
+
+    protected StateGraphException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
+        super(message, cause, enableSuppression, writableStackTrace);
+    }
+}
diff --git a/src/main/java/edu/kit/formal/interpreter/ControlFlowNode.java b/src/main/java/edu/kit/formal/interpreter/graphs/ControlFlowNode.java
similarity index 88%
rename from src/main/java/edu/kit/formal/interpreter/ControlFlowNode.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/ControlFlowNode.java
index 529e4f6fd85613859e56d42dc317424ef811496d..7692e153e04fff2968eece81ef81ef223e3505aa 100644
--- a/src/main/java/edu/kit/formal/interpreter/ControlFlowNode.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/ControlFlowNode.java
@@ -1,4 +1,4 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
 
 import edu.kit.formal.proofscriptparser.ast.ASTNode;
 
@@ -6,7 +6,7 @@ import java.util.LinkedList;
 import java.util.List;
 
 /**
- * ControlFlowNode for ControlFlowGraph to look up step-edges for teh debugger.
+ * ControlFlowNode for ControlFlowGraph to look up step-edges for the debugger.
  */
 
 public class ControlFlowNode {
@@ -47,7 +47,7 @@ public class ControlFlowNode {
         StringBuilder sb = new StringBuilder();
         sb.append("Node {");
         if (scriptstmt != null) {
-            sb.append(scriptstmt.getNodeName().toString() + "@" + scriptstmt.getStartPosition());
+            sb.append(scriptstmt.getNodeName()).append("@").append(scriptstmt.getStartPosition());
         } else {
             sb.append("No Stmt");
         }
diff --git a/src/main/java/edu/kit/formal/interpreter/EdgeTypes.java b/src/main/java/edu/kit/formal/interpreter/graphs/EdgeTypes.java
similarity index 52%
rename from src/main/java/edu/kit/formal/interpreter/EdgeTypes.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/EdgeTypes.java
index af433117f02ed483042f448f9ef3940713a62e44..14f657bac8b5791afda0a5b7ae5d7c31e5e7903c 100644
--- a/src/main/java/edu/kit/formal/interpreter/EdgeTypes.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/EdgeTypes.java
@@ -1,7 +1,7 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
 
 /**
- * Created by sarah on 6/20/17.
+ * Edge Types a state graph and control flow graph may have
  */
 public enum EdgeTypes {
     STEP_INTO, STEP_OVER, STEP_BACK, STEP_RETURN, STEP_OVER_COND, STATE_FLOW;
diff --git a/src/main/java/edu/kit/formal/interpreter/GraphChangedListener.java b/src/main/java/edu/kit/formal/interpreter/graphs/GraphChangedListener.java
similarity index 63%
rename from src/main/java/edu/kit/formal/interpreter/GraphChangedListener.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/GraphChangedListener.java
index 602a163034aac5cffa24855e36d54240e6c90d18..2eb2cfea2be41e616ed65c80bf2c0d962e0dce1d 100644
--- a/src/main/java/edu/kit/formal/interpreter/GraphChangedListener.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/GraphChangedListener.java
@@ -1,4 +1,6 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
+
+import edu.kit.formal.interpreter.NodeAddedEvent;
 
 /**
  * Listener for Change events in the state graph
diff --git a/src/main/java/edu/kit/formal/interpreter/PTreeNode.java b/src/main/java/edu/kit/formal/interpreter/graphs/PTreeNode.java
similarity index 94%
rename from src/main/java/edu/kit/formal/interpreter/PTreeNode.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/PTreeNode.java
index b3c4919a89e7150520c6d981eb0d5031cadcd8bc..ac6e57779b298967b6c5c06825f40efaa63a7c9b 100644
--- a/src/main/java/edu/kit/formal/interpreter/PTreeNode.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/PTreeNode.java
@@ -1,4 +1,4 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
 
 import edu.kit.formal.interpreter.data.KeyData;
 import edu.kit.formal.interpreter.data.State;
@@ -7,7 +7,7 @@ import edu.kit.formal.proofscriptparser.ast.ASTNode;
 import java.util.LinkedList;
 
 /**
- * Inner class representing nodes in the graph
+ * Inner class representing nodes in the stategraph graph
  * A node contains a reference to the ASTNode and a reference to the corresponding interpreter state if this state is already interpreted, null otherwise.
  */
 public class PTreeNode {
diff --git a/src/main/java/edu/kit/formal/interpreter/ProgramFlowVisitor.java b/src/main/java/edu/kit/formal/interpreter/graphs/ProgramFlowVisitor.java
similarity index 99%
rename from src/main/java/edu/kit/formal/interpreter/ProgramFlowVisitor.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/ProgramFlowVisitor.java
index 1015ae811ed4152ca7a7bab9d406c47e0b3da0ad..12d4cac8204c2f2b98c42c08cb78548a50cb5a2a 100644
--- a/src/main/java/edu/kit/formal/interpreter/ProgramFlowVisitor.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/ProgramFlowVisitor.java
@@ -1,4 +1,4 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
 
 import com.google.common.graph.EndpointPair;
 import com.google.common.graph.MutableValueGraph;
diff --git a/src/main/java/edu/kit/formal/interpreter/ProofTreeController.java b/src/main/java/edu/kit/formal/interpreter/graphs/ProofTreeController.java
similarity index 97%
rename from src/main/java/edu/kit/formal/interpreter/ProofTreeController.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/ProofTreeController.java
index 59f1f02babc93d60dc19fef6b71458e9976fe558..47b7039fe4e56b2d0d5bb961b52c28f3ef7a35f9 100644
--- a/src/main/java/edu/kit/formal/interpreter/ProofTreeController.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/ProofTreeController.java
@@ -1,6 +1,8 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
 
 import edu.kit.formal.gui.controller.PuppetMaster;
+import edu.kit.formal.interpreter.Interpreter;
+import edu.kit.formal.interpreter.InterpretingService;
 import edu.kit.formal.interpreter.data.GoalNode;
 import edu.kit.formal.interpreter.data.KeyData;
 import edu.kit.formal.interpreter.data.State;
@@ -162,8 +164,7 @@ public class ProofTreeController {
      */
     public PTreeNode stepBack() {
         PTreeNode current = statePointer;
-        PTreeNode backState = stateGraphVisitor.getStepBack(current);
-        this.statePointer = backState;
+        this.statePointer = stateGraphVisitor.getStepBack(current);
         setNewState(statePointer.getState());
         return statePointer;
 
diff --git a/src/main/java/edu/kit/formal/interpreter/StateGraphVisitor.java b/src/main/java/edu/kit/formal/interpreter/graphs/StateGraphVisitor.java
similarity index 92%
rename from src/main/java/edu/kit/formal/interpreter/StateGraphVisitor.java
rename to src/main/java/edu/kit/formal/interpreter/graphs/StateGraphVisitor.java
index 76be23f220e02b5da517fd79e0a3a3dbf79a7b2f..5b0791514f960baf6d023163a3abb9bd5e597cf1 100644
--- a/src/main/java/edu/kit/formal/interpreter/StateGraphVisitor.java
+++ b/src/main/java/edu/kit/formal/interpreter/graphs/StateGraphVisitor.java
@@ -1,10 +1,13 @@
-package edu.kit.formal.interpreter;
+package edu.kit.formal.interpreter.graphs;
 
 
 import com.google.common.graph.MutableValueGraph;
 import com.google.common.graph.ValueGraphBuilder;
+import edu.kit.formal.interpreter.Interpreter;
+import edu.kit.formal.interpreter.NodeAddedEvent;
 import edu.kit.formal.interpreter.data.KeyData;
 import edu.kit.formal.interpreter.data.State;
+import edu.kit.formal.interpreter.exceptions.StateGraphException;
 import edu.kit.formal.proofscriptparser.DefaultASTVisitor;
 import edu.kit.formal.proofscriptparser.ast.*;
 import javafx.application.Platform;
@@ -19,7 +22,7 @@ import java.util.Set;
 
 /**
  * State graph that is computed on the fly while stepping through script
- * A Node in the graph is a PTreeNode {@link edu.kit.formal.interpreter.PTreeNode}
+ * A Node in the graph is a PTreeNode {@link PTreeNode}
  * Edges are computed on the fly while
  */
 public class StateGraphVisitor extends DefaultASTVisitor<Void> {
@@ -85,10 +88,13 @@ public class StateGraphVisitor extends DefaultASTVisitor<Void> {
     public void createRootNode(ASTNode node) {
         PTreeNode newStateNode = new PTreeNode(node);
         newStateNode.setState(currentInterpreter.getCurrentState().copy());
-        stateGraph.addNode(newStateNode);
-        this.root.set(newStateNode);
-        lastNode = newStateNode;
-
+        boolean res = stateGraph.addNode(newStateNode);
+        if (!res) {
+            throw new StateGraphException("Could not create new state for ASTNode " + node + " and add it to the stategraph");
+        } else {
+            this.root.set(newStateNode);
+            lastNode = newStateNode;
+        }
     }
 
     public void setUpGraph() {
@@ -161,8 +167,7 @@ public class StateGraphVisitor extends DefaultASTVisitor<Void> {
      */
     public Void createNewNode(ASTNode node) {
         State<KeyData> lastState = lastNode.getState();
-        //maybe delete
-        //lastNode.setState(currentInterpreter.getCurrentState());
+
         PTreeNode newStateNode;
         newStateNode = new PTreeNode(node);
 
@@ -273,7 +278,7 @@ public class StateGraphVisitor extends DefaultASTVisitor<Void> {
     }
 
 
-    protected void fireNodeAdded(NodeAddedEvent nodeAddedEvent) {
+    private void fireNodeAdded(NodeAddedEvent nodeAddedEvent) {
         changeListeners.forEach(list -> {
             Platform.runLater(() -> {
                 list.graphChanged(nodeAddedEvent);
diff --git a/src/test/java/edu/kit/formal/interpreter/ProgramFlowVisitorTest.java b/src/test/java/edu/kit/formal/interpreter/ProgramFlowVisitorTest.java
index edb13a2310c71aeea780e7d302f296bad9dd655e..ff9c3d5448ce86dd6f06d9be8aa6024a91a30e96 100644
--- a/src/test/java/edu/kit/formal/interpreter/ProgramFlowVisitorTest.java
+++ b/src/test/java/edu/kit/formal/interpreter/ProgramFlowVisitorTest.java
@@ -1,6 +1,7 @@
 package edu.kit.formal.interpreter;
 
 import edu.kit.formal.interpreter.funchdl.DefaultLookup;
+import edu.kit.formal.interpreter.graphs.ProgramFlowVisitor;
 import edu.kit.formal.proofscriptparser.Facade;
 import edu.kit.formal.proofscriptparser.ScriptLanguageParser;
 import edu.kit.formal.proofscriptparser.TransformAst;