src/Pure/Thy/thy_syntax.scala

     {
       val doc_edits = new mutable.ListBuffer[Document.Edit_Command]
       var nodes = previous.nodes
 
       edits foreach {
-        case (name, Document.Node.Remove()) =>
-          doc_edits += (name -> Document.Node.Remove())
+        case (name, Document.Node.Clear()) =>
+          doc_edits += (name -> Document.Node.Clear())
           nodes -= name
 
         case (name, Document.Node.Edits(text_edits)) =>
           val node = nodes(name)
           val commands0 = node.commands