diff --git a/src/main/java/de/neemann/digital/core/arithmetic/Add.java b/src/main/java/de/neemann/digital/core/arithmetic/Add.java
index 2a3b7dddc..16803697c 100644
--- a/src/main/java/de/neemann/digital/core/arithmetic/Add.java
+++ b/src/main/java/de/neemann/digital/core/arithmetic/Add.java
@@ -12,10 +12,15 @@ import de.neemann.digital.core.element.ElementTypeDescription;
 import static de.neemann.digital.core.element.PinInfo.input;
 
 /**
+ * A adder.
+ *
  * @author hneemann
  */
 public class Add extends Node implements Element {
 
+    /**
+     * The adders description
+     */
     public static final ElementTypeDescription DESCRIPTION
             = new ElementTypeDescription(Add.class, input("a"), input("b"), input("c_i"))
             .addAttribute(AttributeKey.Rotate)
@@ -24,50 +29,47 @@ public class Add extends Node implements Element {
 
     private final int bits;
     private final ObservableValue sum;
-    private final ObservableValue c_out;
+    private final ObservableValue cOut;
     private final long mask;
     protected ObservableValue a;
     protected ObservableValue b;
-    protected ObservableValue c_in;
+    protected ObservableValue cIn;
     protected long value;
 
+    /**
+     * Create a new instance
+     *
+     * @param attributes the attributes
+     */
     public Add(ElementAttributes attributes) {
         bits = attributes.get(AttributeKey.Bits);
         this.mask = 1 << bits;
 
         this.sum = new ObservableValue("s", bits);
-        this.c_out = new ObservableValue("c_o", 1);
+        this.cOut = new ObservableValue("c_o", 1);
     }
 
     @Override
     public void readInputs() throws NodeException {
-        value = a.getValue() + b.getValue() + c_in.getValue();
+        value = a.getValue() + b.getValue() + cIn.getValue();
     }
 
     @Override
     public void writeOutputs() throws NodeException {
         sum.setValue(value);
-        c_out.setValue((value & mask) == 0 ? 0 : 1);
-    }
-
-    public ObservableValue getSum() {
-        return sum;
-    }
-
-    public ObservableValue getCOut() {
-        return c_out;
+        cOut.setValue((value & mask) == 0 ? 0 : 1);
     }
 
     @Override
     public void setInputs(ObservableValue... inputs) throws BitsException {
         a = inputs[0].addObserverToValue(this).checkBits(bits, this);
         b = inputs[1].addObserverToValue(this).checkBits(bits, this);
-        c_in = inputs[2].addObserverToValue(this).checkBits(1, this);
+        cIn = inputs[2].addObserverToValue(this).checkBits(1, this);
     }
 
     @Override
     public ObservableValue[] getOutputs() {
-        return new ObservableValue[]{sum, c_out};
+        return new ObservableValue[]{sum, cOut};
     }
 
 }
diff --git a/src/main/java/de/neemann/digital/core/arithmetic/Comparator.java b/src/main/java/de/neemann/digital/core/arithmetic/Comparator.java
index ddb31acbb..1990840a5 100644
--- a/src/main/java/de/neemann/digital/core/arithmetic/Comparator.java
+++ b/src/main/java/de/neemann/digital/core/arithmetic/Comparator.java
@@ -12,10 +12,14 @@ import de.neemann.digital.core.element.ElementTypeDescription;
 import static de.neemann.digital.core.element.PinInfo.input;
 
 /**
+ * A comparator
  * @author hneemann
  */
 public class Comparator extends Node implements Element {
 
+    /**
+     * The comparators description
+     */
     public static final ElementTypeDescription DESCRIPTION = new ElementTypeDescription(Comparator.class, input("a"), input("b"))
             .addAttribute(AttributeKey.Rotate)
             .addAttribute(AttributeKey.Label)
@@ -32,8 +36,14 @@ public class Comparator extends Node implements Element {
     private final int maskOr;
     private ObservableValue a;
     private ObservableValue b;
-    private long valueA, valueB;
+    private long valueA;
+    private long valueB;
 
+    /**
+     * Create a new instance
+     *
+     * @param attributes the attributes
+     */
     public Comparator(ElementAttributes attributes) {
         signed = attributes.get(AttributeKey.Signed);
         bits = attributes.get(AttributeKey.Bits);
@@ -66,8 +76,8 @@ public class Comparator extends Node implements Element {
             }
 
             boolean kl = valueA < valueB;
-            aklb.setValue(kl ? 1 : 0);
-            agrb.setValue(kl ? 0 : 1);
+            aklb.setBool(kl);
+            agrb.setBool(!kl);
         }
     }
 
diff --git a/src/main/java/de/neemann/digital/core/arithmetic/Mul.java b/src/main/java/de/neemann/digital/core/arithmetic/Mul.java
index 62e480e50..14b8f1f94 100644
--- a/src/main/java/de/neemann/digital/core/arithmetic/Mul.java
+++ b/src/main/java/de/neemann/digital/core/arithmetic/Mul.java
@@ -11,10 +11,15 @@ import de.neemann.digital.core.element.ElementTypeDescription;
 import static de.neemann.digital.core.element.PinInfo.input;
 
 /**
+ * A multiplicator
+ *
  * @author hneemann
  */
 public class Mul extends Node implements Element {
 
+    /**
+     * The multiplicators description
+     */
     public static final ElementTypeDescription DESCRIPTION
             = new ElementTypeDescription(Mul.class, input("a"), input("b"))
             .addAttribute(AttributeKey.Rotate)
@@ -27,6 +32,11 @@ public class Mul extends Node implements Element {
     private ObservableValue b;
     private long value;
 
+    /**
+     * Creates a new instance
+     *
+     * @param attributes the attributes
+     */
     public Mul(ElementAttributes attributes) {
         bits = attributes.get(AttributeKey.Bits);
         this.mul = new ObservableValue("mul", bits * 2);
@@ -42,10 +52,6 @@ public class Mul extends Node implements Element {
         mul.setValue(value);
     }
 
-    public ObservableValue getMul() {
-        return mul;
-    }
-
     @Override
     public void setInputs(ObservableValue... inputs) throws NodeException {
         a = inputs[0].addObserverToValue(this).checkBits(bits, this);
diff --git a/src/main/java/de/neemann/digital/core/arithmetic/Sub.java b/src/main/java/de/neemann/digital/core/arithmetic/Sub.java
index 28a71e712..f650d1c19 100644
--- a/src/main/java/de/neemann/digital/core/arithmetic/Sub.java
+++ b/src/main/java/de/neemann/digital/core/arithmetic/Sub.java
@@ -12,18 +12,26 @@ import static de.neemann.digital.core.element.PinInfo.input;
  */
 public class Sub extends Add {
 
+    /**
+     * The subtractors description
+     */
     public static final ElementTypeDescription DESCRIPTION
             = new ElementTypeDescription(Sub.class, input("a"), input("b"), input("c_i"))
             .addAttribute(AttributeKey.Rotate)
             .addAttribute(AttributeKey.Label)
             .addAttribute(AttributeKey.Bits);
 
+    /**
+     * Creates a new instance
+     *
+     * @param attributes the attributes
+     */
     public Sub(ElementAttributes attributes) {
         super(attributes);
     }
 
     @Override
     public void readInputs() throws NodeException {
-        value = a.getValue() - b.getValue() - c_in.getValue();
+        value = a.getValue() - b.getValue() - cIn.getValue();
     }
 }
diff --git a/src/main/java/de/neemann/digital/core/arithmetic/package-info.java b/src/main/java/de/neemann/digital/core/arithmetic/package-info.java
new file mode 100644
index 000000000..2126058e3
--- /dev/null
+++ b/src/main/java/de/neemann/digital/core/arithmetic/package-info.java
@@ -0,0 +1,6 @@
+/**
+ * Simple arithmetic operations like addition, subtraction and multiplication
+ *
+ * @author hneemann
+ */
+package de.neemann.digital.core.arithmetic;
diff --git a/src/test/java/de/neemann/digital/core/arithmetic/MulTest.java b/src/test/java/de/neemann/digital/core/arithmetic/MulTest.java
index 6616ef89e..18e8ef5d9 100644
--- a/src/test/java/de/neemann/digital/core/arithmetic/MulTest.java
+++ b/src/test/java/de/neemann/digital/core/arithmetic/MulTest.java
@@ -19,7 +19,7 @@ public class MulTest extends TestCase {
         Mul node = model.add(new Mul(new ElementAttributes().setBits(4)));
         node.setInputs(a, b);
 
-        TestExecuter sc = new TestExecuter(model).setInputs(a, b).setOutputs(node.getMul());
+        TestExecuter sc = new TestExecuter(model).setInputs(a, b).setOutputsOf(node);
         sc.check(0, 0, 0);
         sc.check(6, 6, 36);
         sc.check(15, 15, 225);
diff --git a/src/test/java/de/neemann/digital/core/arithmetic/SubTest.java b/src/test/java/de/neemann/digital/core/arithmetic/SubTest.java
index 5d20feb10..3cd3a98db 100644
--- a/src/test/java/de/neemann/digital/core/arithmetic/SubTest.java
+++ b/src/test/java/de/neemann/digital/core/arithmetic/SubTest.java
@@ -22,7 +22,7 @@ public class SubTest extends TestCase {
         node.setInputs(a, b, c);
         model.add(node);
 
-        TestExecuter sc = new TestExecuter(model).setInputs(a, b, c).setOutputs(node.getSum(), node.getCOut());
+        TestExecuter sc = new TestExecuter(model).setInputs(a, b, c).setOutputsOf(node);
         sc.check(0, 0, 0, 0, 0);
         sc.check(3, 2, 0, 1, 0);
         sc.check(2, 3, 0, 15, 1);