/*
   * Allow downgrade
   * The normal solver operation tries to install (to update to) the 'best' package,
   * usually the one with the highest version.
   * If allow_downgrade is set, packages may be downgraded in order to
   * fulfill a request or a dependency
   * call-seq:
   *  solver.allow_downgrade -> bool
   *
   */
  int allow_downgrade()