Department of Computer Science, School of Mathematics, Statistics, and Computer Science, University of Tehran, P. O. Box: 14155-6455, Tehran, Iran, Laboratoire dInformatique (LIX), Ecole Polytechnique, 91128 Palaiseau CEDEX, France and School of Biological Science, Institute for Research in Fundamental Sciences (IPM), P.O. Box: 19395-5746 Tehran, Iran.|Department of Computer Science, School of Mathematics, Statistics, and Computer Science, University of Tehran, P. O. Box: 14155-6455, Tehran, Iran, Laboratoire dInformatique (LIX), Ecole Polytechnique, 91128 Palaiseau CEDEX, France and School of Biological Science, Institute for Research in Fundamental Sciences (IPM), P.O. Box: 19395-5746 Tehran, Iran.|Department of Computer Science, School of Mathematics, Statistics, and Computer Science, University of Tehran, P. O. Box: 14155-6455, Tehran, Iran, Laboratoire dInformatique (LIX), Ecole Polytechnique, 91128 Palaiseau CEDEX, France and School of Biological Science, Institute for Research in Fundamental Sciences (IPM), P.O. Box: 19395-5746 Tehran, Iran.