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abstract class List[+a] {
def head: a
def tail: List[a]
def cons[b >: a](x: b): List[b] = new Cons[b, a](x, this)
}
object Nil extends List[Nothing] {
def error(msg: String): Nothing = throw new java.lang.Error(msg)
def head: Nothing = error("Nil.head")
def tail: List[Nothing] = error("Nil.tail")
}
class Cons[c, d <: c](x: c, xs: List[d]) extends List[c] {
def head: c = x
def tail: List[c] = xs
}
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