diff -r 2bf8cfc98c4d -r 9c1173a7e4cb src/Tools/VSCode/src/document_model.scala
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/Tools/VSCode/src/document_model.scala	Mon Dec 19 20:27:49 2016 +0100
@@ -0,0 +1,58 @@
+/*  Title:      Tools/VSCode/src/document_model.scala
+    Author:     Makarius
+
+Document model for line-oriented text.
+*/
+
+package isabelle.vscode
+
+
+import isabelle._
+
+import scala.util.parsing.input.CharSequenceReader
+
+
+case class Document_Model(
+  session: Session, doc: Line.Document, node_name: Document.Node.Name,
+  changed: Boolean = true)
+{
+  /* header */
+
+  def is_theory: Boolean = node_name.is_theory
+
+  lazy val node_header: Document.Node.Header =
+    if (is_theory) {
+      val toks = Token.explode(Thy_Header.bootstrap_syntax.keywords, doc.text)
+      val toks1 = toks.dropWhile(tok => !tok.is_command(Thy_Header.THEORY))
+      toks1.iterator.zipWithIndex.collectFirst({ case (tok, i) if tok.is_begin => i }) match {
+        case Some(i) =>
+          session.resources.check_thy_reader("", node_name,
+            new CharSequenceReader(toks1.take(i + 1).map(_.source).mkString), Token.Pos.command)
+        case None => Document.Node.no_header
+      }
+    }
+    else Document.Node.no_header
+
+
+  /* edits */
+
+  def text_edits: List[Text.Edit] =
+    if (changed) List(Text.Edit.insert(0, doc.text)) else Nil
+
+  def node_edits: List[Document.Edit_Text] =
+    if (changed) {
+      List(session.header_edit(node_name, node_header),
+        node_name -> Document.Node.Clear(),
+        node_name -> Document.Node.Edits(text_edits),
+        node_name ->
+          Document.Node.Perspective(true, Text.Perspective.full, Document.Node.Overlays.empty))
+    }
+    else Nil
+
+  def unchanged: Document_Model = if (changed) copy(changed = false) else this
+
+
+  /* snapshot */
+
+  def snapshot(): Document.Snapshot = session.snapshot(node_name, text_edits)
+}