/*
   * Include (specific) solvable
   *
   * Including a Solvable explicitly means that this Solvable _must_ be installed.
   *
   * Including a Solvable by name means that one Solvable
   * with the given name must be installed. The solver is free to
   * choose one.
   *
   * Including a Solvable by relation means that any Solvable
   * providing the given relation must be installed.
   * 
   * call-seq:
   *  solver.include(solvable)
   *  solver.include("kernel")
   *  solver.include(relation)
   *
   */
  void include( XSolvable *xs )