Nedarim, Chapter Three, Mishnah Six
This mishnah and the remainder of the chapter deal with a person who vowed that he would not benefit from a certain type of person. These mishnayoth provide definitions who are those types of people.
He who vows [not to benefit] from seafarers, may benefit from land-dwellers;
[But he who vows not to benefit] from land-dwellers, is forbidden [to benefit] even from seafarers, because seafarers are included in land-dwellers; not those who merely travel from Acco to Jaffa, but even those who sail away great distances [from land].
This mishnah teaches that the term seafarers does not include land-dwellers, and therefore if one vows not to benefit from seafarers he may derive benefit from land-dwellers. However, the term land-dwellers does include seafarers, because all seafarers occasionally come to land. This is obviously true for seafarers who make short journeys from Acco to Jaffa, but even for those who make long journeys. Therefore, if he makes a vow not to benefit from land-dwellers, he cannot derive benefit even from seafarers.