src/Doc/ROOT

--- a/src/Doc/ROOT	Thu Aug 17 14:13:34 2017 +0200
+++ b/src/Doc/ROOT	Thu Aug 17 14:28:01 2017 +0200
@@ -298,6 +298,8 @@
 session Logics_ZF (doc) in "Logics_ZF" = ZF +
   options [document_variants = "logics-ZF", print_mode = "brackets",
     thy_output_source]
+  sessions
+    FOL
   theories
     IFOL_examples
     FOL_examples