You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: diagram.rb
    path of file: ./repos/genometools/gtruby/annotationsketch
    
    the code of the file until where you have to start completion: #
# Copyright (c) 2007-2008 Gordon Gremme <gordon@gremme.org>
# Copyright (c) 2007-2008 Center for Bioinformatics, University of Hamburg
#
# Permission to use, copy, modify, and distribute this software for any
# purpose with or without fee is hereby granted, provided that the above
# copyright notice and this permission notice appear in all copies.
#
# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL W
    
    def initialize(ptr)
      @diagram = ptr
      @diagram.free = GT::symbol("gt_diagram_delete", "0P")
#      callback_releaser = Proc.new do
#        DL.remove_callback(@tsf) unless @tsf.nil
#      end
#      ObjectSpace.define_finalizer(self, callback_releaser)
    end

    def set_track_selector_func(proc)
      @tsf = DL.callback('0PPP') do |b_ptr, string_ptr, data_ptr|
               b = GT::Block.new(b_ptr)
               string = GT::Str.new(string_ptr)
               s = proc.call(b, data_ptr)
               if (s.nil?) then
                 GT::gterror("Track selector callback must return a string!")
               else
                 string.append_cstr(s)
               end
             end
      GT.gt_diagram_set_track_selector_func(@diagram, @tsf, GT::NULL)
    end