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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 _downgrade_v4(op):
 """ Downgrades assets db by copying the `exchange_full` column to `exchange`, then dropping the `exchange_full` column. """

    op.drop_index('ix_equities_fuzzy_symbol')
    op.drop_index('ix_equities_company_symbol')

    op.execute("UPDATE equities SET exchange = exchange_full")

    with op.batch_alter_table('equities') as batch_op:
        batch_op.drop_column('exchange_full')

    op.create_index('ix_equities_fuzzy_symbol',
                    table_name='equities',
                    columns=['fuzzy_symbol'])
    op.create_index('ix_equities_company_symbol',
                    table_name='equities',
                    columns=['company_symbol'])