diff --git a/src/Pure/Admin/build_vampire.scala b/src/Pure/Admin/build_vampire.scala
--- a/src/Pure/Admin/build_vampire.scala
+++ b/src/Pure/Admin/build_vampire.scala
@@ -1,167 +1,169 @@
 /*  Title:      Pure/Admin/build_vampire.scala
     Author:     Makarius
 
 Build Isabelle Vampire component from repository.
 */
 
 package isabelle
 
 
 object Build_Vampire
 {
   val default_repository = "https://github.com/vprover/vampire.git"
   val default_version1 = "4.5.1"
   val default_version2 = "df87588848db"
   val default_jobs = 1
 
   def make_component_name(version: String) = "vampire-" + version
 
 
   /* build Vampire */
 
   def build_vampire(
     repository: String = default_repository,
     version1: String = default_version1,
     version2: String = default_version2,
     jobs: Int = default_jobs,
     component_name: String = "",
     verbose: Boolean = false,
     progress: Progress = new Progress,
     target_dir: Path = Path.current)
   {
     Isabelle_System.with_tmp_dir("build")(tmp_dir =>
     {
       /* component and platform */
 
       val component = proper_string(component_name) getOrElse make_component_name(version1)
       val component_dir = Isabelle_System.new_directory(target_dir + Path.basic(component))
       progress.echo("Component " + component_dir)
 
       val platform_name =
         proper_string(Isabelle_System.getenv("ISABELLE_PLATFORM64")) getOrElse
           error("No 64bit platform")
       val platform_dir = Isabelle_System.make_directory(component_dir + Path.basic(platform_name))
 
 
       /* clone repository */
 
       progress.echo("Cloning repository " + repository)
       progress.bash("git clone " + Bash.string(repository) + " vampire",
         cwd = tmp_dir.file, echo = verbose).check
 
       val source_dir = tmp_dir + Path.explode("vampire")
 
       File.copy(source_dir + Path.explode("LICENCE"), component_dir)
 
 
       /* build versions */
 
       for { (rev, exe) <- List(version1 -> "vampire", version2 -> "vampire_polymorphic") } {
         progress.echo("Building " + exe + " (rev " + rev + ")")
         progress.bash("git checkout --quiet --detach " + Bash.string(rev),
           cwd = source_dir.file, echo = verbose).check
 
-        val build_dir = Isabelle_System.make_directory(source_dir + Path.explode("build"))
+        val build_dir = source_dir + Path.explode("build")
+        Isabelle_System.rm_tree(build_dir)
+        Isabelle_System.make_directory(build_dir)
 
         val cmake_opts = if (Platform.is_linux) "-DBUILD_SHARED_LIBS=0 " else ""
         val cmake_out =
           progress.bash("cmake " + cmake_opts + """-G "Unix Makefiles" ..""",
             cwd = build_dir.file, echo = verbose).check.out
 
         val Pattern = """-- Setting binary name to '?([^\s']*)'?""".r
         val binary =
           split_lines(cmake_out).collectFirst({ case Pattern(name) => name })
             .getOrElse(error("Failed to determine binary name from cmake output:\n" + cmake_out))
 
         progress.bash("make -j" + jobs, cwd = build_dir.file, echo = verbose).check
 
         File.copy(build_dir + Path.basic("bin") + Path.basic(binary).platform_exe,
           platform_dir + Path.basic(exe).platform_exe)
       }
 
 
       /* settings */
 
       val etc_dir = Isabelle_System.make_directory(component_dir + Path.basic("etc"))
       File.write(etc_dir + Path.basic("settings"),
         """# -*- shell-script -*- :mode=shellscript:
 
 VAMPIRE_HOME="$COMPONENT/$ISABELLE_PLATFORM64"
 
 ISABELLE_VAMPIRE="$VAMPIRE_HOME/vampire"
 ISABELLE_VAMPIRE_POLYMORPHIC="$VAMPIRE_HOME/vampire_polymorphic"
 
 VAMPIRE_EXTRA_OPTIONS=""
 """)
 
 
       /* README */
 
       File.write(component_dir + Path.basic("README"),
         "This Isabelle component provides two versions of Vampire from\n" + repository + """
 
   * vampire: standard version (regular stable release)
   * vampire_polymorphic: special version for polymorphic FOL and HOL
     (intermediate repository version)
 
 The executables have been built like this:
 
     git checkout COMMIT
     cmake .
     make
 
 The precise commit id is revealed by executing "vampire --version".
 
 
         Makarius
         """ + Date.Format.date(Date.now()) + "\n")
     })
   }
 
 
   /* Isabelle tool wrapper */
 
   val isabelle_tool =
     Isabelle_Tool("build_vampire", "build prover component from repository", Scala_Project.here,
     args =>
     {
       var target_dir = Path.current
       var repository = default_repository
       var version1 = default_version1
       var version2 = default_version2
       var jobs = default_jobs
       var component_name = ""
       var verbose = false
 
       val getopts = Getopts("""
 Usage: isabelle build_vampire [OPTIONS]
 
   Options are:
     -D DIR       target directory (default ".")
     -U URL       repository (default: """" + default_repository + """")
     -V REV1      standard version (default: """" + default_version1 + """")
     -W REV2      polymorphic version (default: """" + default_version2 + """")
     -j NUMBER    parallel jobs for make (default: """ + default_jobs + """)
     -n NAME      component name (default: """" + make_component_name("REV1") + """")
     -v           verbose
 
   Build prover component from official download.
 """,
         "D:" -> (arg => target_dir = Path.explode(arg)),
         "U:" -> (arg => repository = arg),
         "V:" -> (arg => version1 = arg),
         "W:" -> (arg => version2 = arg),
         "j:" -> (arg => jobs = Value.Nat.parse(arg)),
         "n:" -> (arg => component_name = arg),
         "v" -> (_ => verbose = true))
 
       val more_args = getopts(args)
       if (more_args.nonEmpty) getopts.usage()
 
       val progress = new Console_Progress()
 
       build_vampire(repository = repository, version1 = version1, version2 = version2,
         component_name = component_name, jobs = jobs, verbose = verbose, progress = progress,
         target_dir = target_dir)
     })
 }