List.isEmpty in Leon lib

library/Option.scala

@@ -32,6 +32,7 @@ sealed abstract class Option[T] {
   def nonEmpty  = !isEmpty
 
   def isDefined = !isEmpty
+
 }
 
 case class Some[T](v: T) extends Option[T]

library/collection/List.scala

@@ -285,6 +285,11 @@ sealed abstract class List[T] {
     }
   }
 
+  def isEmpty = this match { 
+    case Nil() => true
+    case _ => false 
+  }
+
 }
 
 @ignore