from __future__ import annotations
import types
from typing import List, Generator

class Prefix:

    # FIXME ‹list_in› should be iterable, not list
    def __init__( self, list_in: List[ int ] ) -> None:
        self.slice = 0
        self.list = list_in
        self.lenlist = len( list_in )

    def __iter__( self ) -> Prefix:
        return self

    def __next__( self ) -> List[ int ]:
        slice_ = self.slice
        self.slice += 1
        if slice_ > self.lenlist:
            raise StopIteration
        return self.list[ 0 : slice_ ]

class Sum:
    def __init__( self, list_in: List[ int ] ) -> None:
        self.prefix = Prefix( list_in )
        next( self.prefix )

    def __iter__( self ) -> Sum: return self
    def __next__( self ) -> int:
        return sum( next( self.prefix ) )


def prefixes( list_in: List[ int ] ) -> Prefix:
    return Prefix( list_in )

def prefix_sum( list_in: List[ int ] ) -> Sum:
    return Sum( list_in )

import run_tests