use Set::IntSpan qw(grep_set map_set);
  
  $Set::IntSpan::Empty_String = $string;
  
  $set    = new   Set::IntSpan $set_spec;
  $valid  = valid Set::IntSpan $run_list;
  $set    = copy  $set $set_spec;
  
  $run_list = run_list $set;
  @elements = elements $set;
  @spans    = spans    $set;
  
  $u_set = union      $set $set_spec;
  $i_set = intersect  $set $set_spec;
  $x_set = xor        $set $set_spec;
  $d_set = diff       $set $set_spec;
  $c_set = complement $set;

# and many more methods