diff --git a/src/Pure/Admin/build_doc.scala b/src/Pure/Admin/build_doc.scala
--- a/src/Pure/Admin/build_doc.scala
+++ b/src/Pure/Admin/build_doc.scala
@@ -1,94 +1,91 @@
 /*  Title:      Pure/Admin/build_doc.scala
     Author:     Makarius
 
 Build Isabelle documentation.
 */
 
 package isabelle
 
 
-import java.io.{File => JFile}
-
-
 object Build_Doc
 {
   /* build_doc */
 
   def build_doc(
     options: Options,
     progress: Progress = new Progress,
     all_docs: Boolean = false,
     max_jobs: Int = 1,
     docs: List[String] = Nil)
   {
     val store = Sessions.store(options)
 
     val sessions_structure = Sessions.load_structure(options)
     val selected =
       for {
         session <- sessions_structure.build_topological_order
         info = sessions_structure(session)
         if info.groups.contains("doc")
         doc = info.options.string("document_variants")
         if all_docs || docs.contains(doc)
       } yield (doc, session)
 
     val documents = selected.map(_._1)
     val selection = Sessions.Selection(sessions = selected.map(_._2))
 
     docs.filter(doc => !documents.contains(doc)) match {
       case Nil =>
       case bad => error("No documentation session for " + commas_quote(bad))
     }
 
     progress.echo("Build started for sessions " + commas_quote(selection.sessions))
     Build.build(options, selection = selection, progress = progress, max_jobs = max_jobs).ok ||
       error("Build failed")
 
     progress.echo("Build started for documentation " + commas_quote(documents))
     val doc_options =
       options + "document=pdf" + "document_output=~~/doc" + "document_output_sources=false"
     val deps = Sessions.load_structure(doc_options).selection_deps(selection)
     for (session <- selection.sessions) {
       progress.expose_interrupt()
       Present.build_documents(session, deps, store, progress = progress)
     }
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("build_doc", "build Isabelle documentation", args =>
     {
       var all_docs = false
       var max_jobs = 1
       var options = Options.init()
 
       val getopts =
         Getopts("""
 Usage: isabelle build_doc [OPTIONS] [DOCS ...]
 
   Options are:
     -a           select all documentation sessions
     -j INT       maximum number of parallel jobs (default 1)
     -o OPTION    override Isabelle system OPTION (via NAME=VAL or NAME)
 
   Build Isabelle documentation from documentation sessions with
   suitable document_variants entry.
 """,
           "a" -> (_ => all_docs = true),
           "j:" -> (arg => max_jobs = Value.Int.parse(arg)),
           "o:" -> (arg => options = options + arg))
 
       val docs = getopts(args)
 
       if (!all_docs && docs.isEmpty) getopts.usage()
 
       val progress = new Console_Progress()
 
       progress.interrupt_handler {
         build_doc(options, progress, all_docs, max_jobs, docs)
       }
     })
 }