def _index_to_row_col(lines, index):
    r"""
    >>> lines = ['hello\n', 'world\n']
    >>> _index_to_row_col(lines, 0)
    (0, 0)
    >>> _index_to_row_col(lines, 7)
    (1, 1)
    """
    if index < 0:
        raise IndexError('negative index')
    current_index = 0
    for line_number, line in enumerate(lines):
        line_length = len(line)
        if current_index + line_length > index:
            return line_number, index - current_index
        current_index += line_length
    raise IndexError('index %d out of range' % index)