def merge_next(letters: list, positions: list) -> list:
    """Given a list of letter positions, merge each letter with its next neighbor.
    >>> merge_next(['a', 'b', 'o', 'v', 'o' ], [0, 2])
    ['ab', '', 'ov', '', 'o']
    >>> # Note: because it operates on the original list passed in, the effect is not cummulative:
    >>> merge_next(['a', 'b', 'o', 'v', 'o' ], [0, 2, 3])
    ['ab', '', 'ov', 'o', '']
    """
    for pos in positions:
        letters[pos] = letters[pos] + letters[pos + 1]
        letters[pos + 1] = ""
    return letters