The union can circumvent a time window by agreeing to a new contract before the window opens thus moving the window to the end of the new contract, when they can move it again.

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They have to be clean and sober for at least 30 days and sign a contract agreeing not to use drugs, be in the presence of known drug users or frequent known drug houses and they must attend three meetings a week.

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Campbell sued, invoking a contract O'Neill had signed agreeing not to join a competitor for 18 months after leaving Campbell.

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In Laredo, Texas, school officials are considering asking next year's seniors to sign a contract agreeing graduation ceremonies are a privilege that can be revoked for bad behavior, said Veronica Castillon, a spokeswoman for the Laredo Independent School District.

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Most firms do, Paige says, agreeing as a matter of contract, for example, not to pass on the cost of training summer associates or overhead as defined by the client.

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The 29-year-old is yet to start a match for the U's since agreeing a short-term contract over the summer.

BBC: Striker Wayne Gray happy to wait for U's start

Yates, who is out of contract at the end of the season, is close to agreeing a new deal at Vale Park.

BBC: Lack of goals cost Port Vale a play-off place - Yates

Since then, the former Everton man has been under intense scrutiny, first announcing he wanted to leave Old Trafford but then agreeing a new five-year multi-million pound contract at the club two days later.

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With his contract due to expire this summer, Kroldrup extended his stay at Fiorentina by agreeing a new two-year deal in May.

BBC: Sport - World Cup 2010 - per_kroldrup