diff --git a/prism/src/prism/ModelVariablesDD.java b/prism/src/prism/ModelVariablesDD.java
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+//==============================================================================
+//	
+//	Copyright (c) 2015-
+//	Authors:
+//	* Joachim Klein <klein@tcs.inf.tu-dresden.de> (TU Dresden)
+//	
+//------------------------------------------------------------------------------
+//	
+//	This file is part of PRISM.
+//	
+//	PRISM is free software; you can redistribute it and/or modify
+//	it under the terms of the GNU General Public License as published by
+//	the Free Software Foundation; either version 2 of the License, or
+//	(at your option) any later version.
+//	
+//	PRISM is distributed in the hope that it will be useful,
+//	but WITHOUT ANY WARRANTY; without even the implied warranty of
+//	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//	GNU General Public License for more details.
+//	
+//	You should have received a copy of the GNU General Public License
+//	along with PRISM; if not, write to the Free Software Foundation,
+//	Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+//	
+//==============================================================================
+
+package prism;
+
+import java.util.Vector;
+
+import jdd.JDD;
+import jdd.JDDNode;
+import jdd.JDDVars;
+
+/**
+ * Management and information storage about the DD variables
+ * used by a symbolic model.
+ * <br>
+ * After use, the variable references should be cleared by
+ * calling {@code clear()}.
+ */
+public class ModelVariablesDD
+{
+	/** Vector of DD variable names for the allocated variables */
+	private Vector<String> ddVarNames = new Vector<String>();
+	/** The allocated variables */
+	private JDDVars ddVariables = new JDDVars();
+	/** The next free variable index */
+	private int nextDDVarIndex = 0;
+
+	/** Storage for the preallocated extra state variables */
+	private JDDVars extraStateVariables = new JDDVars();
+
+	/** Constructor */
+	public ModelVariablesDD()
+	{
+	}
+
+	/**
+	 * Create a copy of this ModelVariablesDD.
+	 * All variable information is copied, i.e., the
+	 * result has to be cleared after use.
+	 */
+	public ModelVariablesDD copy()
+	{
+		ModelVariablesDD result = new ModelVariablesDD();
+		result.ddVarNames = new Vector<String>(ddVarNames);
+		result.ddVariables = ddVariables.copy();
+		result.extraStateVariables = extraStateVariables.copy();
+		result.nextDDVarIndex = nextDDVarIndex;
+
+		return result;
+	}
+
+	/** Clear all variable references */
+	public void clear()
+	{
+		ddVariables.derefAll();
+		extraStateVariables.derefAll();
+	}
+
+	/** Return the vector with variable names for a given variable index */
+	public Vector<String> getDDVarNames()
+	{
+		return ddVarNames;
+	}
+
+	/**
+	 * Allocate a new variable for the model.
+	 * <br>[ REFS: <i>result</i> ]
+	 * @param name a name for the variable
+	 */
+	public JDDNode allocateVariable(String name)
+	{
+		JDDNode v = JDD.Var(nextDDVarIndex);
+		nextDDVarIndex++;
+
+		ddVariables.addVar(v.copy());
+
+		ddVarNames.add(name);
+		return v;
+	}
+
+	/** Preallocate the given number of state variables */
+	public void preallocateExtraStateVariables(int count)
+	{
+		for (int i=0; i<count; i++) {
+			JDDNode v = allocateVariable("");
+			extraStateVariables.addVar(v);
+		}
+	}
+
+	/**
+	 * Allocate a new state variable from the pool of
+	 * preallocated state variables.
+	 * <br>
+	 * If the pool is empty, allocate a new variable
+	 * at the end.
+	 * <br> [ REFS: <i>result</i> ]
+	 * @param name the name for the variable
+	 */
+	public JDDNode allocateExtraStateVariable(String name)
+	{
+		JDDNode v;
+		if (extraStateVariables.getNumVars() > 0) {
+			v = extraStateVariables.getVar(0);
+			extraStateVariables.removeVar(v);
+
+			ddVarNames.set(v.getIndex(), name);
+		} else {
+			// allocate at the end
+			v = allocateVariable(name);
+		}
+		return v;
+	}
+
+	/**
+	 * Returns true if there are enough preallocated state variables
+	 * such that {@code 2*bits} variables can be allocated from the pool
+	 */
+	public boolean canPrependExtraStateVariable(int bits)
+	{
+		return (bits*2 <= extraStateVariables.getNumVars());
+	}
+
+	/**
+	 * Allocate {@code bits} pairs of row and col variables.
+	 * <br>
+	 * If {@code prepend} is true and there is not enough space
+	 * in the preallocated state variable pool, throws an exception.
+	 * <br>[ REFS: <i>result</i> ]
+	 * @param bits number of state variable pairs
+	 * @param name the name of the variable
+	 * @param prepend should the variables be allocated from the preallocated pool?
+	 */
+	public JDDVars allocateExtraStateVariable(int bits, String name, boolean prepend) throws PrismException
+	{
+		int newVarCount = 2*bits;
+
+		JDDVars result = new JDDVars();
+		if (newVarCount == 0) return result;
+
+		if (prepend) {
+			if (!canPrependExtraStateVariable(bits)) {
+				throw new PrismException("Can not prepend "+(newVarCount)+" extra row/col variables");
+			}
+
+			// take the bottom nVarCount extra variables from the
+			// extraStateVariables array, starting with index v
+			int v = extraStateVariables.getNumVars() - newVarCount;
+			for (int i=0; i < bits; i++) {
+				// transfer row, col from extraStateVariables to result,
+				// no need to ref again
+				JDDNode row_var = extraStateVariables.getVar(v++);
+				JDDNode col_var = extraStateVariables.getVar(v++);
+
+				result.addVar(row_var);
+				result.addVar(col_var);
+
+				ddVarNames.set(row_var.getIndex(), name+"."+i);
+				ddVarNames.set(col_var.getIndex(), name+"'."+i);
+			}
+
+			// remove variables from extraStateVariables
+			extraStateVariables.removeVars(result);
+
+			return result;
+		} else {
+			for (int i=0; i < bits; i++) {
+				result.addVar(allocateVariable(name+"."+i));
+				result.addVar(allocateVariable(name+"'."+i));
+			}
+		}
+
+		return result;
+	}
+
+	/**
+	 * Get the extra state variables.
+	 * This is not a copy.
+	 *
+	 * <br>[ REFs: <i>none</i>, DEREFs: <i>none</i> ]
+	 */
+	public JDDVars getExtraStateVariables() {
+		return extraStateVariables;
+	}
+
+}