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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[All] {
def error(msg: String): All = throw new java.lang.Error(msg);
def head: All = error("Nil.head");
def tail: List[All] = 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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