>>> 1 in [1, 2, 3]
True

class In(a: Any) {
  def in[T](s: Seq[T]) = s.contains(a)
}
implicit def any2In(a : Any) = new In(a)
 
scala> "a" in List(1, 2, "a")
res0: Boolean = true
 
scala> "a" in List(1, 2, "b")
res1: Boolean = false