/*
   * Exclude (specific) solvable
   *
   * Excluding a (specific) Solvable means that this Solvable _must_ _not_
   * be installed.
   *
   * Excluding a Solvable by name means that any Solvable
   * with the given name must not be installed.
   *
   * Excluding a Solvable by relation means that any Solvable
   * providing the given relation must not be installed.
   *
   * call-seq:
   *  solver.exclude(solvable)
   *  solver.exclude("mono-core")
   *  solver.exclude(relation)
   *
   */
  void exclude( XSolvable *xs )