<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _find_line_start_index(self, index):
 """ For the index of a character at a certain line, calculate the index of the first character on that line. Return (row, index) tuple. """

        indexes = self._line_start_indexes

        pos = bisect.bisect_right(indexes, index) - 1
        return pos, indexes[pos]