/*
   * Lock request
   *
   * Ensure solvable stays installed/uninstalled
   * * 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.lock(solvable) -> void
   *  request.lock("kernel") -> void
   *  request.lock(relation) -> void
   *
   */
  void lock( XSolvable *xs )