diff --git a/src/de/dhbwstuttgart/typeinference/unify/interfaces/IUnifier.java b/src/de/dhbwstuttgart/typeinference/unify/interfaces/IUnifier.java
index ca0d6ce2..da18fa8f 100644
--- a/src/de/dhbwstuttgart/typeinference/unify/interfaces/IUnifier.java
+++ b/src/de/dhbwstuttgart/typeinference/unify/interfaces/IUnifier.java
@@ -1,9 +1,9 @@
 package de.dhbwstuttgart.typeinference.unify.interfaces;
 
-import java.util.Optional;
+import java.util.function.Function;
 
 import de.dhbwstuttgart.typinference.unify.model.MPair;
 
-public interface IUnifier {
-	public Optional<MPair> apply(MPair pair);
+public interface IUnifier extends Function<MPair, MPair>{
+
 }
diff --git a/src/de/dhbwstuttgart/typeinference/unifynew/Unify.java b/src/de/dhbwstuttgart/typeinference/unifynew/Unify.java
index 2047b150..1313b6fa 100644
--- a/src/de/dhbwstuttgart/typeinference/unifynew/Unify.java
+++ b/src/de/dhbwstuttgart/typeinference/unifynew/Unify.java
@@ -13,8 +13,11 @@ import de.dhbwstuttgart.typeinference.Pair;
 import de.dhbwstuttgart.typeinference.exceptions.NotImplementedException;
 import de.dhbwstuttgart.typeinference.unify.interfaces.IFiniteClosure;
 import de.dhbwstuttgart.typeinference.unify.interfaces.IRuleSet;
+import de.dhbwstuttgart.typeinference.unify.interfaces.IUnifier;
 import de.dhbwstuttgart.typinference.unify.model.MPair;
 import de.dhbwstuttgart.typinference.unify.model.PlaceholderType;
+import de.dhbwstuttgart.typinference.unify.model.SimpleType;
+import de.dhbwstuttgart.typinference.unify.model.Type;
 
 /**
  * Implementierung des Unifikationsalgorithmus.
@@ -142,4 +145,11 @@ public class Unify {
 		
 		collection.add(pair2);
 	}
+	
+	private void test() {
+		Type t = new PlaceholderType("T");
+		Type subst = new SimpleType("Subst");
+		
+		IUnifier u = x -> x.substitute(t, subst);
+	}
 }
diff --git a/src/de/dhbwstuttgart/typinference/unify/model/MPair.java b/src/de/dhbwstuttgart/typinference/unify/model/MPair.java
index 0088d02a..9a76f2a9 100644
--- a/src/de/dhbwstuttgart/typinference/unify/model/MPair.java
+++ b/src/de/dhbwstuttgart/typinference/unify/model/MPair.java
@@ -66,6 +66,23 @@ public class MPair {
 				&& other.getRhsType().equals(rhs);
 	}
 	
+	/**
+	 * Substitutes the occurrences of Type t on the left or right side of the pair with the Type subst.
+	 * @param t Type to be replaced.
+	 * @param subst The type replacing t.
+	 * @return A pair where occurrences of t are replaced by subst.
+	 */
+	public MPair substitute(Type t, Type subst) {
+		Type newlhs = lhs;
+		if(lhs.equals(t)) newlhs = subst;
+		
+		Type newrhs = rhs;
+		if(rhs.equals(t)) newrhs = subst;
+		
+		if(newlhs == lhs && newrhs == rhs) return this;
+		return new MPair(newlhs, newrhs, pairOp);
+	}
+	
 	@Override
 	public int hashCode() {
 		return 17 + 31 * lhs.hashCode() + 31 * rhs.hashCode() + 31 * pairOp.hashCode();
diff --git a/test/unify/UnifyTest.java b/test/unify/UnifyTest.java
new file mode 100644
index 00000000..747374b2
--- /dev/null
+++ b/test/unify/UnifyTest.java
@@ -0,0 +1,7 @@
+package unify;
+
+import de.dhbwstuttgart.typeinference.unifynew.Unify;
+
+public class UnifyTest extends Unify {
+
+}