/*
   * Update request
   *
   * Ensure update of a solvable by either
   * * specifying it directly
   * * specify it by name
   * * specify a required relation
   *
   * Except when specified directly, the solver is free to choose any
   * solvable matching the request (by name, by relation)
   *
   * call-seq:
   *  request.update(solvable) -> void
   *  request.update("kernel") -> void
   *  request.update(relation) -> void
   *
   */
  void update( XSolvable *xs )