Department of Computer Science, Freie Universität Berlin, Takustr, 9, 14195 Berlin, Germany. birte.kehr@fu-berlin.de.|Department of Computer Science, Freie Universität Berlin, Takustr, 9, 14195 Berlin, Germany. birte.kehr@fu-berlin.de.|Department of Computer Science, Freie Universität Berlin, Takustr, 9, 14195 Berlin, Germany. birte.kehr@fu-berlin.de.|Section for Computational and RNA Biology, Department of Biology, University of Copenhagen, Ole Maaloes Vej 5, 2200 Copenhagen, Denmark. krogh@binf.ku.dk.