  The simulation of genealogical trees backwards in time, from observations up
to the most recent common ancestor (MRCA), is hindered by the fact that, while
approaching the root of the tree, coalescent events become rarer, with a
corresponding increase in computation time. The recently proposed "Time
Machine" tackles this issue by stopping the simulation of the tree before
reaching the MRCA and correcting for the induced bias. We present a
computationally efficient implementation of this approach that exploits
multithreading.
