WarehouseYieldChange effects - Where is their definition?

[Jeppetto]

<Modifier id="AKHET_MOD_NAVIGABLE_RIVER_FOOD" collection="COLLECTION_PLAYER_CITIES" effect="EFFECT_CITY_GRANT_WAREHOUSE_YIELD">
            <Argument name="WarehouseYieldChange">EgyptAkhetNavigableRiverFood</Argument>
            <String context="Description">LOC_TRADITION_AKHET_DESCRIPTION</String>
</Modifier>

Does anyone know where is the EgyptAkhetNavigableRiverFood effect defined? Presumably where all Warehouse effects are but I cannot find such a place.