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.

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()])