import M4a._


val f_urban = (x:(Int, Int)) => if (x._1 > 3) Some(List(x)) else None

assert(first(List((1,0),(2,0),(3,0),(4,0)), f_urban) == Some(List((4,0))))
assert(first(List((1,0),(2,0),(3,0)), f_urban) == None)