<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def insert(self, index, text):
 """Insert line to the document """

        if index < 0 or index > self._doc.blockCount():
            raise IndexError('Invalid block index', index)

        if index == 0:  # first
            cursor = QTextCursor(self._doc.firstBlock())
            cursor.insertText(text)
            cursor.insertBlock()
        elif index != self._doc.blockCount():  # not the last
            cursor = QTextCursor(self._doc.findBlockByNumber(index).previous())
            cursor.movePosition(QTextCursor.EndOfBlock)
            cursor.insertBlock()
            cursor.insertText(text)
        else:  # last append to the end
            self.append(text)