Reza Pulungan Holger Hermanns Effective Minimization of Acyclic Phase-Type Representations. 128-143 2008 ASMTA http://dx.doi.org/10.1007/978-3-540-68982-9_10 conf/asmta/2008 db/conf/asmta/asmta2008.html#PulunganH08