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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 _generate_file_set(self, var=None, start_date=None, end_date=None, domain=None, intvl_in=None, dtype_in_vert=None, dtype_in_time=None, intvl_out=None):
 """Returns the file_set for the given interval in."""

        try:
            return self.file_map[intvl_in]
        except KeyError:
            raise KeyError('File set does not exist for the specified'
                           ' intvl_in {0}'.format(intvl_in))