code: def which(program):
      
      if os.path.split(program)[0]:
          program_path = find_exe(program)
          if program_path:
              return program_path
      else:
          for path in get_path_list():
              program_path = find_exe(os.path.join(path, program))
              if program_path:
                  return program_path
      return None
docs: Identify the location of an executable file.