The sentences that contain an example of contested usage are:
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"The chair for the chair of the board of directors is at the head of the board table." (The term "chair" can be contested as it can refer to a physical object or a position in an organization.)
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"The postman needs to know who this letter is addressed to so it can be delivered." (The use of "who" versus "whom" can be contested here, as traditional grammar rules suggest "whom" should be used in formal contexts.)
Both of these examples illustrate areas of language usage that may be debated or challenged.