code: def pack(field: str, kwargs: Dict[str, Any],
           default: Optional[Any] = None, sep: str=',') -> str:
      
      if default is not None:
          value = kwargs.get(field, default)
      else:
          value = kwargs[field]
      if isinstance(value, str):
          return value
      elif isinstance(value, collections.abc.Iterable):
          return sep.join(str(f) for f in value)
      else:
          return str(value)
docs: Util for joining multiple fields with commas