diff -r 83a2b6976515 -r c3589f2dff31 src/Pure/Thy/thy_syntax.scala
--- a/src/Pure/Thy/thy_syntax.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/Thy/thy_syntax.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -140,7 +140,7 @@
             edit_text(es, insert_text(Some(cmd), e.text))
 
           case None =>
-            require(e.is_insert && e.start == 0)
+            require(e.is_insert && e.start == 0, "bad text edit")
             edit_text(es, insert_text(None, e.text))
         }
       case Nil => commands