diff --git a/prism/src/prism/ExplicitFiles2MTBDD.java b/prism/src/prism/ExplicitFiles2MTBDD.java
index 857b5085..6f82be51 100644
--- a/prism/src/prism/ExplicitFiles2MTBDD.java
+++ b/prism/src/prism/ExplicitFiles2MTBDD.java
@@ -431,10 +431,8 @@ public class ExplicitFiles2MTBDD
 		moduleDDColVars[0] = new JDDVars();
 		// go thru all variables
 		for (i = 0; i < numVars; i++) {
-			varDDRowVars[i].refAll();
-			varDDColVars[i].refAll();
-			moduleDDRowVars[0].addVars(varDDRowVars[i]);
-			moduleDDColVars[0].addVars(varDDColVars[i]);
+			moduleDDRowVars[0].copyVarsFrom(varDDRowVars[i]);
+			moduleDDColVars[0].copyVarsFrom(varDDColVars[i]);
 		}
 
 		// put refs for all vars in whole system together
@@ -450,10 +448,8 @@ public class ExplicitFiles2MTBDD
 		// go thru all variables
 		for (i = 0; i < numVars; i++) {
 			// add to list
-			varDDRowVars[i].refAll();
-			varDDColVars[i].refAll();
-			allDDRowVars.addVars(varDDRowVars[i]);
-			allDDColVars.addVars(varDDColVars[i]);
+			allDDRowVars.copyVarsFrom(varDDRowVars[i]);
+			allDDColVars.copyVarsFrom(varDDColVars[i]);
 		}
 		if (modelType == ModelType.MDP) {
 			for (i = 0; i < ddChoiceVars.length; i++) {
diff --git a/prism/src/prism/ExplicitModel2MTBDD.java b/prism/src/prism/ExplicitModel2MTBDD.java
index 351b076c..357969b1 100644
--- a/prism/src/prism/ExplicitModel2MTBDD.java
+++ b/prism/src/prism/ExplicitModel2MTBDD.java
@@ -327,10 +327,8 @@ public class ExplicitModel2MTBDD
 		moduleDDColVars[0] = new JDDVars();
 		// go thru all variables
 		for (i = 0; i < numVars; i++) {
-			varDDRowVars[i].refAll();
-			varDDColVars[i].refAll();
-			moduleDDRowVars[0].addVars(varDDRowVars[i]);
-			moduleDDColVars[0].addVars(varDDColVars[i]);
+			moduleDDRowVars[0].copyVarsFrom(varDDRowVars[i]);
+			moduleDDColVars[0].copyVarsFrom(varDDColVars[i]);
 		}
 
 		// put refs for all vars in whole system together
@@ -346,10 +344,8 @@ public class ExplicitModel2MTBDD
 		// go thru all variables
 		for (i = 0; i < numVars; i++) {
 			// add to list
-			varDDRowVars[i].refAll();
-			varDDColVars[i].refAll();
-			allDDRowVars.addVars(varDDRowVars[i]);
-			allDDColVars.addVars(varDDColVars[i]);
+			allDDRowVars.copyVarsFrom(varDDRowVars[i]);
+			allDDColVars.copyVarsFrom(varDDColVars[i]);
 		}
 		if (modelType == ModelType.MDP) {
 			for (i = 0; i < ddChoiceVars.length; i++) {
diff --git a/prism/src/prism/Modules2MTBDD.java b/prism/src/prism/Modules2MTBDD.java
index c8ed71fe..c10ec384 100644
--- a/prism/src/prism/Modules2MTBDD.java
+++ b/prism/src/prism/Modules2MTBDD.java
@@ -566,19 +566,17 @@ public class Modules2MTBDD
 		}
 		// go thru all variables
 		for (i = 0; i < numVars; i++) {
-			varDDRowVars[i].refAll();
-			varDDColVars[i].refAll();
 			// check which module it belongs to
 			m = varList.getModule(i);
 			// if global...
 			if (m == -1) {
-				globalDDRowVars.addVars(varDDRowVars[i]);
-				globalDDColVars.addVars(varDDColVars[i]);
+				globalDDRowVars.copyVarsFrom(varDDRowVars[i]);
+				globalDDColVars.copyVarsFrom(varDDColVars[i]);
 			}
 			// otherwise...
 			else {
-				moduleDDRowVars[m].addVars(varDDRowVars[i]);
-				moduleDDColVars[m].addVars(varDDColVars[i]);
+				moduleDDRowVars[m].copyVarsFrom(varDDRowVars[i]);
+				moduleDDColVars[m].copyVarsFrom(varDDColVars[i]);
 			}
 		}
 		
@@ -595,10 +593,8 @@ public class Modules2MTBDD
 		// go thru all variables
 		for (i = 0; i < numVars; i++) {
 			// add to list
-			varDDRowVars[i].refAll();
-			varDDColVars[i].refAll();
-			allDDRowVars.addVars(varDDRowVars[i]);
-			allDDColVars.addVars(varDDColVars[i]);
+			allDDRowVars.copyVarsFrom(varDDRowVars[i]);
+			allDDColVars.copyVarsFrom(varDDColVars[i]);
 		}
 		if (modelType == ModelType.MDP) {
 			// go thru all syncronising action vars