def METHOD_NAME(start_idx, count, initial_string):
    """
    Return the substring of length `count` starting at `start_idx`.
    :param start_idx:               starting index of the substring
    :param count:                   length of the substring in bytes
    :param initial_string:          original string
    :return:                        the substring
    """
    new_value = initial_string.value[start_idx.value : start_idx.value + count.value]
    return StringV(new_value)