pplications requiring non-FIFO queuing schemes.
  However, Boost.Statechart will probably provide a
  <code>priority_scheduler</code> in the future so that custom schedulers
  need to be implemented only in rare cases.</p>

  <h2><a name="MemberFunctionsVsFunctionObjects" id=
  "MemberFunctionsVsFunctionObjects">User actions: Member functions vs.
  function objects</a></h2>

  <p>All user-supplied functions (<code>react</code> member functions,
  entry-, exit- and transition-actions) must be class members. 