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object typers {
class List[+T] {
def :: (x: T) = new :: (x, this)
def len: Int = this match {
case x :: xs1 => 1 + xs1.len
case Nil => 0
}
}
object Nil extends List[Nothing]
case class :: [+T] (hd: T, tl: List[T]) extends List[T]
def len[U](xs: List[U]): Int = xs match {
case x :: xs1 => 1 + len(xs1)
case Nil => 0
}
}
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