diff --git a/src/main/java/de/vill/model/constraint/AndConstraint.java b/src/main/java/de/vill/model/constraint/AndConstraint.java
index 7587150..a67e709 100644
--- a/src/main/java/de/vill/model/constraint/AndConstraint.java
+++ b/src/main/java/de/vill/model/constraint/AndConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.AutomaticBrackets;
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -36,7 +37,7 @@ public void setRight(Constraint right){
     @Override
     public String toString(boolean withSubmodels, String currentAlias) {
         return AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, left, withSubmodels, currentAlias) +
-                " & " +
+                " " + ConstantSymbols.AND + " " +
                 AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, right, withSubmodels, currentAlias);
     }
 
diff --git a/src/main/java/de/vill/model/constraint/EqualEquationConstraint.java b/src/main/java/de/vill/model/constraint/EqualEquationConstraint.java
index 796c64e..734bfcf 100644
--- a/src/main/java/de/vill/model/constraint/EqualEquationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/EqualEquationConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.VariableReference;
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -9,7 +10,7 @@
 public class EqualEquationConstraint extends ExpressionConstraint {
 
     public EqualEquationConstraint(final Expression left, final Expression right) {
-        super(left, right, "==");
+        super(left, right, ConstantSymbols.EQUALS);
     }
 
     @Override
diff --git a/src/main/java/de/vill/model/constraint/EquivalenceConstraint.java b/src/main/java/de/vill/model/constraint/EquivalenceConstraint.java
index 6a07fee..2a61220 100644
--- a/src/main/java/de/vill/model/constraint/EquivalenceConstraint.java
+++ b/src/main/java/de/vill/model/constraint/EquivalenceConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.AutomaticBrackets;
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -28,7 +29,7 @@ public Constraint getRight() {
     @Override
     public String toString(boolean withSubmodels, String currentAlias) {
         return AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, left, withSubmodels, currentAlias) +
-                " <=> " +
+                " " + ConstantSymbols.EQUIVALENT + " " +
                 AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, right, withSubmodels, currentAlias);
     }
 
diff --git a/src/main/java/de/vill/model/constraint/ExpressionConstraint.java b/src/main/java/de/vill/model/constraint/ExpressionConstraint.java
index 056dede..d7bd28f 100644
--- a/src/main/java/de/vill/model/constraint/ExpressionConstraint.java
+++ b/src/main/java/de/vill/model/constraint/ExpressionConstraint.java
@@ -3,6 +3,7 @@
 import de.vill.model.Feature;
 import de.vill.model.building.VariableReference;
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
 
 import java.util.*;
 
@@ -75,17 +76,17 @@ public boolean evaluate(Set<Feature> selectedFeatures) {
             return false;
         }
 
-        if ("==".equals(expressionSymbol)) {
+        if (ConstantSymbols.EQUALS.equals(expressionSymbol)) {
             return leftResult == rightResult;
-        } else if ("<".equals(expressionSymbol)) {
+        } else if (ConstantSymbols.LOWER.equals(expressionSymbol)) {
             return leftResult < rightResult;
-        } else if (">".equals(expressionSymbol)) {
+        } else if (ConstantSymbols.GREATER.equals(expressionSymbol)) {
             return leftResult > rightResult;
-        } else if (">=".equals(expressionSymbol)) {
+        } else if (ConstantSymbols.GREATER_OR_EQUAL.equals(expressionSymbol)) {
             return leftResult >= rightResult;
-        } else if ("<=".equals(expressionSymbol)) {
+        } else if (ConstantSymbols.LOWER_OR_EQUAL.equals(expressionSymbol)) {
             return leftResult <= rightResult;
-        } else if ("!=".equals(expressionSymbol)) {
+        } else if (ConstantSymbols.NOT_EQUALS.equals(expressionSymbol)) {
             return leftResult != rightResult;
         }
         return false;
diff --git a/src/main/java/de/vill/model/constraint/GreaterEqualsEquationConstraint.java b/src/main/java/de/vill/model/constraint/GreaterEqualsEquationConstraint.java
index 4aca930..2b357b6 100644
--- a/src/main/java/de/vill/model/constraint/GreaterEqualsEquationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/GreaterEqualsEquationConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.VariableReference;
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.Collections;
@@ -9,7 +10,7 @@
 
 public class GreaterEqualsEquationConstraint extends ExpressionConstraint {
     public GreaterEqualsEquationConstraint(final Expression left, final Expression right) {
-        super(left, right, ">=");
+        super(left, right, ConstantSymbols.GREATER_OR_EQUAL);
     }
 
     @Override
diff --git a/src/main/java/de/vill/model/constraint/GreaterEquationConstraint.java b/src/main/java/de/vill/model/constraint/GreaterEquationConstraint.java
index e323cd8..9dada1b 100644
--- a/src/main/java/de/vill/model/constraint/GreaterEquationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/GreaterEquationConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.VariableReference;
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.Collections;
@@ -9,7 +10,7 @@
 
 public class GreaterEquationConstraint extends ExpressionConstraint {
     public GreaterEquationConstraint(final Expression left, final Expression right) {
-        super(left, right, ">");
+        super(left, right, ConstantSymbols.GREATER);
     }
 
     @Override
diff --git a/src/main/java/de/vill/model/constraint/ImplicationConstraint.java b/src/main/java/de/vill/model/constraint/ImplicationConstraint.java
index 854baf3..8b49e30 100644
--- a/src/main/java/de/vill/model/constraint/ImplicationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/ImplicationConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.AutomaticBrackets;
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -28,7 +29,7 @@ public Constraint getRight() {
     @Override
     public String toString(boolean withSubmodels, String currentAlias) {
         return AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, left, withSubmodels, currentAlias) +
-                " => " +
+                " " + ConstantSymbols.IMPLIES + " " +
                 AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, right, withSubmodels, currentAlias);
     }
 
diff --git a/src/main/java/de/vill/model/constraint/LowerEqualsEquationConstraint.java b/src/main/java/de/vill/model/constraint/LowerEqualsEquationConstraint.java
index ab03bcb..0df9bcc 100644
--- a/src/main/java/de/vill/model/constraint/LowerEqualsEquationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/LowerEqualsEquationConstraint.java
@@ -1,12 +1,14 @@
 package de.vill.model.constraint;
 
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
+
 import java.util.Collections;
 import java.util.List;
 
 public class LowerEqualsEquationConstraint extends ExpressionConstraint {
     public LowerEqualsEquationConstraint(final Expression left, final Expression right) {
-        super(left, right, "<=");
+        super(left, right, ConstantSymbols.LOWER_OR_EQUAL);
     }
 
     @Override
diff --git a/src/main/java/de/vill/model/constraint/LowerEquationConstraint.java b/src/main/java/de/vill/model/constraint/LowerEquationConstraint.java
index 4f6f991..e730b87 100644
--- a/src/main/java/de/vill/model/constraint/LowerEquationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/LowerEquationConstraint.java
@@ -1,12 +1,14 @@
 package de.vill.model.constraint;
 
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
+
 import java.util.Collections;
 import java.util.List;
 
 public class LowerEquationConstraint extends ExpressionConstraint {
     public LowerEquationConstraint(final Expression left, final Expression right) {
-        super(left, right, "<");
+        super(left, right, ConstantSymbols.LOWER);
     }
 
     @Override
diff --git a/src/main/java/de/vill/model/constraint/MultiOrConstraint.java b/src/main/java/de/vill/model/constraint/MultiOrConstraint.java
index bb31d49..453a963 100644
--- a/src/main/java/de/vill/model/constraint/MultiOrConstraint.java
+++ b/src/main/java/de/vill/model/constraint/MultiOrConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.AutomaticBrackets;
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.LinkedList;
@@ -20,7 +21,7 @@ public String toString(boolean withSubmodels, String currentAlias) {
         StringBuilder result = new StringBuilder();
         for (Constraint part : sub_parts) {
             result.append(AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, part, withSubmodels, currentAlias));
-            result.append(" | ");
+            result.append(" "+ ConstantSymbols.OR + " ");
         }
         result.delete(result.length() -3, result.length());
         return result.toString();
diff --git a/src/main/java/de/vill/model/constraint/NotConstraint.java b/src/main/java/de/vill/model/constraint/NotConstraint.java
index 6490f0e..197ae50 100644
--- a/src/main/java/de/vill/model/constraint/NotConstraint.java
+++ b/src/main/java/de/vill/model/constraint/NotConstraint.java
@@ -1,6 +1,7 @@
 package de.vill.model.constraint;
 
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.Arrays;
 import java.util.List;
@@ -20,13 +21,13 @@ public Constraint getContent() {
     @Override
     public String toString(boolean withSubmodels, String currentAlias) {
         StringBuilder result = new StringBuilder();
-        result.append("!");
+        result.append(ConstantSymbols.NOT);
         if (content instanceof VariableReference || content instanceof ParenthesisConstraint) {
             result.append(content.toString(withSubmodels, currentAlias));
         } else {
-            result.append("(");
+            result.append(ConstantSymbols.PAREN_OPEN);
             result.append(content.toString(withSubmodels, currentAlias));
-            result.append(")");
+            result.append(ConstantSymbols.PAREN_CLOSE);
         }
         return result.toString();
     }
diff --git a/src/main/java/de/vill/model/constraint/NotEqualsEquationConstraint.java b/src/main/java/de/vill/model/constraint/NotEqualsEquationConstraint.java
index 35470f4..aa4bedb 100644
--- a/src/main/java/de/vill/model/constraint/NotEqualsEquationConstraint.java
+++ b/src/main/java/de/vill/model/constraint/NotEqualsEquationConstraint.java
@@ -1,12 +1,14 @@
 package de.vill.model.constraint;
 
 import de.vill.model.expression.Expression;
+import de.vill.util.ConstantSymbols;
+
 import java.util.Collections;
 import java.util.List;
 
 public class NotEqualsEquationConstraint extends ExpressionConstraint {
     public NotEqualsEquationConstraint(final Expression left, final Expression right) {
-        super(left, right, "!=");
+        super(left, right, ConstantSymbols.NOT_EQUALS);
     }
 
     @Override
diff --git a/src/main/java/de/vill/model/constraint/OrConstraint.java b/src/main/java/de/vill/model/constraint/OrConstraint.java
index b29aba1..0a3c7e8 100644
--- a/src/main/java/de/vill/model/constraint/OrConstraint.java
+++ b/src/main/java/de/vill/model/constraint/OrConstraint.java
@@ -2,6 +2,7 @@
 
 import de.vill.model.building.AutomaticBrackets;
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -37,7 +38,7 @@ public void setRight(Constraint right){
     @Override
     public String toString(boolean withSubmodels, String currentAlias) {
         return AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, left, withSubmodels, currentAlias) +
-                " | " +
+                " " + ConstantSymbols.OR + " " +
                 AutomaticBrackets.enforceConstraintBracketsIfNecessary(this, right, withSubmodels, currentAlias);
     }
 
diff --git a/src/main/java/de/vill/model/constraint/ParenthesisConstraint.java b/src/main/java/de/vill/model/constraint/ParenthesisConstraint.java
index 52b03fe..c37c436 100644
--- a/src/main/java/de/vill/model/constraint/ParenthesisConstraint.java
+++ b/src/main/java/de/vill/model/constraint/ParenthesisConstraint.java
@@ -1,6 +1,7 @@
 package de.vill.model.constraint;
 
 import de.vill.model.building.VariableReference;
+import de.vill.util.ConstantSymbols;
 
 import java.util.Arrays;
 import java.util.List;
@@ -19,9 +20,9 @@ public Constraint getContent() {
 
     @Override
     public String toString(boolean withSubmodels, String currentAlias) {
-        return "(" +
+        return ConstantSymbols.PAREN_OPEN +
                 content.toString(withSubmodels, currentAlias) +
-                ")";
+                ConstantSymbols.PAREN_CLOSE;
     }
 
     @Override
diff --git a/src/main/java/de/vill/util/ConstantSymbols.java b/src/main/java/de/vill/util/ConstantSymbols.java
new file mode 100644
index 0000000..d1b508c
--- /dev/null
+++ b/src/main/java/de/vill/util/ConstantSymbols.java
@@ -0,0 +1,24 @@
+package de.vill.util;
+
+public class ConstantSymbols {
+
+    // Logical operators
+    public static final String OR = "|";
+    public static final String AND = "&";
+    public static final String NOT = "!";
+    public static final String IMPLIES = "=>";
+    public static final String EQUIVALENT = "<=>";
+
+    // Comparison
+    public static final String EQUALS = "==";
+    public static final String NOT_EQUALS = "!=";
+    public static final String GREATER = ">";
+    public static final String LOWER = "<";
+    public static final String GREATER_OR_EQUAL = ">=";
+    public static final String LOWER_OR_EQUAL = "<=";
+
+    //Parenthesis
+    public static final String PAREN_OPEN = "(";
+    public static final String PAREN_CLOSE = ")";
+
+}