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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def find_line_beginning(strings: Sequence[str], linestart: Optional[str]) -> int: """ Finds the index of the line in ``strings`` that begins with ``linestart``, or ``-1`` if none is found. If ``linestart is None``, match an empty line. """

    if linestart is None:  # match an empty line
        for i in range(len(strings)):
            if is_empty_string(strings[i]):
                return i
        return -1
    for i in range(len(strings)):
        if strings[i].find(linestart) == 0:
            return i
    return -1