src/Pure/PIDE/document.scala

         this match {
           case Edits(es) => es.foreach(f)
           case _ =>
         }
       }
+
+      def is_void: Boolean =
+        this match {
+          case Edits(Nil) => true
+          case _ => false
+        }
     }
     case class Clear[A, B]() extends Edit[A, B]
     case class Blob[A, B](blob: Document.Blob) extends Edit[A, B]
 
     case class Edits[A, B](edits: List[A]) extends Edit[A, B]