def carbon_emissions_imports_rule(mod, tx, tmp):
    """
    **Constraint Name**: Carbon_Emissions_Imports_Constraint
    **Defined Over**: CRB_TX_OPR_TMPS

    Constrain the *Import_Carbon_Emissions_Tons* variable to be at least as
    large as the calculated imported carbon emissions for each transmission
    line, based on its CO2-intensity.
    """
    if mod.carbon_cap_zone_import_direction[tx] == "positive":
        return mod.Import_Carbon_Emissions_Tons[tx, tmp] \
            >= mod.Transmit_Power_MW[tx, tmp] \
            * mod.tx_co2_intensity_tons_per_mwh[tx]
    elif mod.carbon_cap_zone_import_direction[tx] == "negative":
        return mod.Import_Carbon_Emissions_Tons[tx, tmp] \
            >= -mod.Transmit_Power_MW[tx, tmp] \
            * mod.tx_co2_intensity_tons_per_mwh[tx]
    else:
        raise ValueError("The parameter carbon_cap_zone_import_direction "
                         "have a value of either 'positive' or "
                         "'negative,' not {}.".format(
                          mod.carbon_cap_zone_import_direction[tx]
                            )
                         )