The module contains various physical time reaction propagation algorithms based on the Gillespie algorithm from 1977. In the current version this module contains - the default version of Gillespie - The modification introduced by Sandmann 2008, which makes possible faster sample reaction arrival times, in case all propensities of the reactions are known exactly. I. e. this modification can be applied, in case propensities were not estimated. - The modification introduced by Petzold et. al. 2006. At the expense of allocating an ancillary propensities array, where their cumulative sum is stored, the velocity of sampling the reaction index increases, dependent on the chosen search policy of assumedSorted ranges. - The module provides also a common testing routine, containing tests for all combinations of modifications.