old_contents: import platform
  from SCons.Script import AddOption, GetOption
  
  SYSTEMS = dict(Linux   = "linux",
  	           Darwin  = "osx",
  	           Windows = "windows")
  
  
  system = str(platform.system())
  if not system in SYSTEMS:
  	system = "unknown"
  
  AddOption('--system',
            dest    = 'system',
            type    = 'string',
            nargs   = 1,
            action  = 'store',
            help    = 'system',
            choices = SYSTEMS.values(),
            default = system)
      
  def generate(env, **kwargs):
      env['SYSTEM'] = GetOption('system')
      if env['SYSTEM'] == 'unknown':
      	raise ValueError('Unknown system')
  
  def exists(env):
      return 1
new_contents: import platform
  from SCons.Script import AddOption, GetOption
  
  SYSTEMS = dict(Linux   = "linux",
  	           Darwin  = "osx",
  	           Windows = "windows")
  
  
  system = str(platform.system())
  if not system in SYSTEMS:
  	system = "unknown"
  
  AddOption('--system',
            dest    = 'system',
            type    = 'choice',
            nargs   = 1,
            action  = 'store',
            help    = 'system',
            choices = SYSTEMS.values(),
            default = system)
      
  def generate(env, **kwargs):
      env['SYSTEM'] = GetOption('system')
      if env['SYSTEM'] == 'unknown':
      	raise ValueError('Unknown system')
  
  def exists(env):
      return 1
subject: Test add site_scons as SCons module