diff -r bb41977edb7e -r a4b430ad848a src/Pure/PIDE/headless.scala
--- a/src/Pure/PIDE/headless.scala	Sun Mar 03 18:45:08 2019 +0100
+++ b/src/Pure/PIDE/headless.scala	Sun Mar 03 19:12:28 2019 +0100
@@ -483,6 +483,18 @@
     def options: Options = session_base_info.options
 
 
+    /* dependencies */
+
+    def used_theories(
+      deps: Sessions.Deps, progress: Progress = No_Progress): List[Document.Node.Name] =
+    {
+      for {
+        (_, name) <- deps.used_theories_condition(options, progress = progress)
+        if !session_base.loaded_theory(name)
+      } yield name
+    }
+
+
     /* session */
 
     def start_session(print_mode: List[String] = Nil, progress: Progress = No_Progress): Session =