"you should know that the said king of portugal has leased this island to christians for ten years, so that no one can enter the bay to trade with the arabs save those who hold the license. these christians have dwellings on the island and factories where they buy and sell with the said arabs who come to the coast to trade for merchandise or various kinds, such as woolen cloths, cotton, silver and coarse cloth, that is cloaks, carpets, and similar articles and above all grain, for they are always short of food. the arabs give in exchange slaves whom the arabs bring from the land of the blacks, and gold dust. the king therefore caused a castle to be built on the island to protect this trade forever. for this reason, portuguese caravels come and go all year long to this island." the licensing of trade described in the passages is an example of which state policy