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import CW8a._
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val f_urban = (x:(Int, Int)) => if (x._1 > 3) Some(List(x)) else None
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assert(first(List((1,0),(2,0),(3,0),(4,0)), f_urban) == Some(List((4,0))))
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assert(first(List((1,0),(2,0),(3,0)), f_urban) == None)