Version 2.4 (???)
* fixed a few bugs in the user guide

Version 2.3 (April 1th, 2013)
* improved reachability graph report
* new feature: let section of transitions
* new feature: description of transitions
* new feature: algorithm delta-ddd (option --algo=delta-ddd)
* option --model-directory can now be used to change the directory
  where helena puts all generated files
* option --collapse is deprecated
* option --static-reductions is deprecated.  use partial-order instead
* enhanced the interactive simulator (command eval)
* fixed a bug caused by name conflicts (when structure had fields named
  left or right)
* fixed a bug caused by state caching reduction

Version 2.2 (October 1th, 2012)
* new feature: inhibitor arcs
* minor bug fixes

Version 2.1 (May 1th, 2012)
*  new feature: net parameters
*  new feature: interactive simulator (--action=simulate)
*  fixed a bug in transition evaluation when input tuples had guards
