src/Pure/General/json.scala

   object Format
   {
     def unapply(s: S): Option[T] =
       try { Some(parse(s, strict = false)) }
       catch { case ERROR(_) => None }
+
+    def apply_lines(json: List[T]): S = json.map(apply).mkString("[", ",\n", "]")
 
     def apply(json: T): S =
     {
       val result = new StringBuilder