/*
   * Return the number of decisions after solving.
   *
   * If its >0, a solution of the Request was found.
   *
   * If its ==0, and 'Solver.problems?' returns +true+, the Request couldn't be solved.
   *
   * If its ==0, and 'Solver.problems?' returns +false+, the Request is trivially solved.
   *
   * call-seq:
   *  solver.decision_count -> int
   *
   */
  int decision_count()