She arrived at school in New York City -- only to learn that an unwritten limit of "two Negro students per year" had already been met.

VOA: special.2010.04.24

Suppose they have a written contract, or even an unwritten contract, what enforces the contract for colluding firms or crooks in jail?

假如他们签了一份书面协议,或干脆口头的,他们怎么强制履行呢

耶鲁公开课 - 博弈论课程节选