t960.scala:18: error: ambiguous reference to overloaded definition,
both method unapply in object List of type [a](xs: List[a])Option[Null]
and  method unapply in object List of type [a](xs: List[a])Option[(a, List[a])]
match argument types (List[a])
    case List(x, xs) => 7
         ^
t960.scala:12: error: method unapply is defined twice
  def unapply[a](xs: List[a]): Option[Null] = xs match {
      ^
two errors found