code: def default_user_agent(name="python-requests"):
      
      _implementation = platform.python_implementation()
  
      if _implementation == 'CPython':
          _implementation_version = platform.python_version()
      elif _implementation == 'PyPy':
          _implementation_version = '%s.%s.%s' % (sys.pypy_version_info.major,
                                                  sys.pypy_version_info.minor,
                                                  sys.pypy_version_info.micro)
          if sys.pypy_version_info.releaselevel != 'final':
              _implementation_version = ''.join([_implementation_version, sys.pypy_version_info.releaselevel])
      elif _implementation == 'Jython':
          _implementation_version = platform.python_version()  # Complete Guess
      elif _implementation == 'IronPython':
          _implementation_version = platform.python_version()  # Complete Guess
      else:
          _implementation_version = 'Unknown'
  
      try:
          p_system = platform.system()
          p_release = platform.release()
      except IOError:
          p_system = 'Unknown'
          p_release = 'Unknown'
  
      return " ".join(['%s/%s' % (name, __version__),
                       '%s/%s' % (_implementation, _implementation_version),
                       '%s/%s' % (p_system, p_release)])
docs: Return a string representing the default user agent.