diff -r c986a422dee1 -r 6c470c918aad src/Pure/System/isabelle_system.scala
--- a/src/Pure/System/isabelle_system.scala	Tue Apr 07 22:13:22 2020 +0200
+++ b/src/Pure/System/isabelle_system.scala	Wed Apr 08 13:14:05 2020 +0200
@@ -54,7 +54,7 @@
     _settings.get
   }
 
-  def init(isabelle_root: String = "", cygwin_root: String = ""): Unit = synchronized {
+  private def init_settings(isabelle_root: String = "", cygwin_root: String = ""): Unit = synchronized {
     if (_settings.isEmpty) {
       val isabelle_root1 =
         bootstrap_directory(isabelle_root, "ISABELLE_ROOT", "isabelle.root", "Isabelle root")
@@ -127,6 +127,12 @@
     }
   }
 
+  def init()
+  {
+    init_settings()
+    services
+  }
+
 
   /* getenv */
 
@@ -383,6 +389,13 @@
   }
 
 
+  /*  user-defined services */
+
+  abstract class Service
+
+  lazy val services: List[Service] = init_classes[Service]("ISABELLE_SCALA_SERVICES")
+
+
   /* default logic */
 
   def default_logic(args: String*): String =