src/Pure/Thy/thy_syntax.scala

     commands: Linear_Set[Command],
     first: Command, last: Command): Linear_Set[Command] =
   {
     val cmds0 = commands.iterator(first, last).toList
     val blobs_spans0 =
-      syntax.parse_spans(syntax.scan(cmds0.iterator.map(_.source).mkString)).
+      syntax.parse_spans(cmds0.iterator.map(_.source).mkString).
         map(span => (Command_Span.resolve_files(resources, syntax, name, span, get_blob), span))
 
     val (cmds1, blobs_spans1) = chop_common(cmds0, blobs_spans0)
 
     val (rev_cmds2, rev_blobs_spans2) = chop_common(cmds1.reverse, blobs_spans1.reverse)