code: def which(program):
      " Check program is exists. "
  
      head, _ = op.split(program)
  
      if head:
          if is_exe(program):
              return program
      else:
          for path in environ["PATH"].split(pathsep):
              exe_file = op.join(path, program)
              if is_exe(exe_file):
                  return exe_file
      return Nonf which(program):
      " Check program is exists. "
  
      head, _ = op.split(program)
  
      if head:
          if is_exe(program):
              return program
      else:
          for path in environ["PATH"].split(pathsep):
              exe_file = op.join(path, program)
              if is_exe(exe_file):
                  return exe_file
      return None
docs: Check program is exists.