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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 line_up_with_epoch(self):
 """Go to the start of the present epoch."""

        if self.parent.notes.annot is None:  # TODO: remove if buttons are disabled
            error_dialog = QErrorMessage()
            error_dialog.setWindowTitle('Error moving to epoch')
            error_dialog.showMessage('No score file loaded')
            error_dialog.exec()
            return

        new_window_start = self.parent.notes.annot.get_epoch_start(
            self.parent.value('window_start'))

        self.parent.overview.update_position(new_window_start)