parameters: (flag_name, flag_values=FLAGS)
docstring: Ensure that flag is not None during program execution.
  
    Registers a flag validator, which will follow usual validator
    rules.
    Args:
      flag_name: string, name of the flag
      flag_values: FlagValues
    Raises:
      AttributeError: if flag_name is not registered as a valid flag name.
docstring_summary: Ensure that flag is not None during program execution.
docstring_tokens: Ensure that flag is not None during program execution .
function: def MarkFlagAsRequired(flag_name, flag_values=FLAGS):
    """Ensure that flag is not None during program execution.
  
    Registers a flag validator, which will follow usual validator
    rules.
    Args:
      flag_name: string, name of the flag
      flag_values: FlagValues
    Raises:
      AttributeError: if flag_name is not registered as a valid flag name.
    """
    RegisterValidator(flag_name,
                      lambda value: value is not None,
                      message='Flag --%s must be specified.' % flag_name,
                      flag_values=flag_values)
function_tokens: def MarkFlagAsRequired ( flag_name , flag_values = FLAGS ) : RegisterValidator ( flag_name , lambda value : value is not None , message = 'Flag --%s must be specified.' % flag_name , flag_values = flag_values )