/*
   * Install request
   *
   * Ensure installation 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.install(solvable) -> void
   *  request.install("kernel") -> void
   *  request.install(relation) -> void
   *
   */
  void install( XSolvable *xs )