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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 _get_report(self, with_line_nums=True):
 		""" 		Returns a report which includes each distinct error only once, together 		with a list of the input lines where the error occurs. The latter will 		be omitted if flag is set to False. 		Helper for the get_report method. 		"""

		templ = '{} ← {}' if with_line_nums else '{}'

		return '\n'.join([
			templ.format(error.string, ','.join(map(str, sorted(set(lines)))))
			for error, lines in self.errors.items()])