docs: def sum_from(zero: T1 = None) -> Callable[[ActualIterable[T1]], T1]:
      """
      >>> from Redy.Collections import Traversal, Flow
      >>> lst: Iterable[int] = [1, 2, 3]
      >>> x = Flow(lst)[Traversal.sum_from(0)].unbox
      >>> assert x is 6
  
      >>> x = Flow(lst)[Traversal.sum_from()].unbox
      >>> assert x is 6
      """
  
      def _(collection: Iterable[T1]) -> T1:
          if zero is None:
              collection = iter(collection)
              _zero = next(collection)
              return builtins.sum(collection, _zero)
  
          return builtins.sum(collection, zero)
  
      return _