clarified empty merge;
tuned;

--- a/src/Pure/Isar/outer_syntax.scala	Sat Oct 07 12:50:05 2017 +0200
+++ b/src/Pure/Isar/outer_syntax.scala	Sat Oct 07 13:13:46 2017 +0200
@@ -18,9 +18,7 @@
 
   def init(): Outer_Syntax = new Outer_Syntax(completion = Completion.init())
 
-  def merge(syns: List[Outer_Syntax]): Outer_Syntax =
-    if (syns.isEmpty) Thy_Header.bootstrap_syntax
-    else (syns.head /: syns.tail)(_ ++ _)
+  def merge(syns: List[Outer_Syntax]): Outer_Syntax = (empty /: syns)(_ ++ _)
 
 
   /* string literals */
@@ -109,6 +107,7 @@
 
   def ++ (other: Outer_Syntax): Outer_Syntax =
     if (this eq other) this
+    else if (this eq Outer_Syntax.empty) other
     else {
       val keywords1 = keywords ++ other.keywords
       val completion1 = completion ++ other.completion

--- a/src/Pure/Thy/thy_info.scala	Sat Oct 07 12:50:05 2017 +0200
+++ b/src/Pure/Thy/thy_info.scala	Sat Oct 07 13:13:46 2017 +0200
@@ -49,12 +49,13 @@
         val name = entry.name.theory
         val imports = entry.header.imports.map(p => p._1.theory)
 
-        val graph1 = (graph /: (name :: imports))(_.default_node(_, Thy_Header.bootstrap_syntax))
+        val graph1 = (graph /: (name :: imports))(_.default_node(_, Outer_Syntax.empty))
         val graph2 = (graph1 /: imports)(_.add_edge(_, name))
 
-        val syntax =
-          Outer_Syntax.merge((name :: graph2.imm_preds(name).toList).map(graph2.get_node(_))) +
-            entry.header
+        val syntax0 = if (name == Thy_Header.PURE) List(Thy_Header.bootstrap_syntax) else Nil
+        val syntax1 = (name :: graph2.imm_preds(name).toList).map(graph2.get_node(_))
+        val syntax = Outer_Syntax.merge(syntax0 ::: syntax1) + entry.header
+
         graph2.map_node(name, _ => syntax)
       })